Biomaterials
Objectives:
This course is designed to provide a general understanding of the
multidisciplinary field of biomaterials. Course biomaterials will rely on general
concepts learned in polymer and biology/biochemistry courses and will further
extend the understanding about the interactions at the interface of material and
biological systems. Current applications of biomaterials will be evaluated to
provide an understanding of material bulk and surface properties, degradation
processes, various biological responses to the materials and the clinical context of
their use. The major classes of materials along with their properties,
characterization, biological responses, and specific clinical applications are
presented and The classes of biomaterials (metals, ceramics, polymers,
semiconductors, composites) used today in medical devices used as implants in
the body or in contact with bodily fluids.
Identify and describe concept of biomedical materials.
Indentify and describe the applications of the biomedical materials such as bone
cement and internal fixation implants in orthopedics.
Apply the relevant methodologies to evaluate the physical, mechanical, optical
and biological properties of currently used or researched biomedical materials.
Apply knowledge of materials science to develop new biomedical materials
Course Topics:
Atomic bonding, crystallography, defects , Diffusion , Mechanical properties,
strengthening mechanisms , Failure of materials and engineering components ,
Phase diagrams , Microstructure design of materials , Polymers , Corrosion ,
Electrical, magnetic, optical properties , Case studies
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Introduction :
Many types of materials are used in biomedical engineering to replace or
supplement natural biological systems. Interaction with blood and tissues is
always of primary importance, but depending on the use of the biomedical
material, mechanical, optical, and transport properties may also be vital.
The most accepted definition of biomaterials is currently the one employed by
the American National Institute of Health that describes biomaterial as ‗‗any
substance or combination of substances, other than drugs, synthetic or natural in
origin, which can be used for any period of time, which augments or replaces
partially or totally any tissue, organ or function of the body, in order to maintain
or improve the quality of life of the individual‘‘.
Biomaterials science: encompasses elements of medicine , biology , chemistry ,
tissue engineering and materials sciences .
Biomaterial science - Definitions
Biomaterials are special materials that have been used for over 50 years in
several medical applications.
A natural or synthetic material (as a metal or polymer) that is suitable for
introduction into living tissue especially as part of a medical device (as an
artificial joint)
Biomaterial : A biomaterial is a nonviable material used in a (medical) device
intended to interact with biological systems - Materials used to safely replace or
interact with biological systems
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A biomaterial is defined as a substance that has been engineered to take a form
which, alone or as part of a complex system, is used to direct, by control of
interactions with components of living systems.
Interdisciplinary interactions are needed
Different disciplines have to work together, starting from the identification of a
need for a biomaterial through development, manufacture, implantation, and
removal from the patient.
some disciplines that intersect
in the development , develop ,
study and application of
biomaterials
include :
biomedical engineer
chemist
chemical engineer
electrical engineer
mechanical engineer
material scientist
biologist
microbiologist
physician
Many Biomaterials are used
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Biomaterials are used in:

















Joint replacements
Bone plates
Bone cement
Artificial ligaments and tendons
Dental implants for tooth fixation
Blood vessel prostheses
Heart valves
Skin repair devices (artificial tissue)
Cochlear replacements
Contact lenses
Breast implants
Drug delivery mechanisms
Sustainable materials
Vascular grafts
Stents
Nerve conduits
Surgical sutures, clips, and staples for wound closure.
Examples of biomaterial applications (USA)
Substitute heart valves (45,000/year)
Artificial hips
(90,000/year)
Dental implants
(275,000/year)
Intraocular lenses
(1.4 millions/year)
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Examples of biomaterial applications
Biomaterials can be in the forms of metals and alloys, ceramics, polymers and
composites. There are several biomaterials utilized for a variety of medical
purposes.
Metals are
used in artificial hips and other joint prostheses and in tooth
implants
Ceramics are used in joint prostheses, in heart valves and tooth replacement .
Polymers are used in eyeglasses, contact lenses, stents, sutures , skin grafts,
porous scaffolds
The major applications include joint replacements, blood vessel prostheses, bone
plates, bone cement, heart valves, artificial ligaments and tendons, dental
implants, skin repair devices, contact lenses and cochlear replacements. The
main issue in the applications of biomaterials is that they must be biocompatible
with the body and mechanically durable, all of which must be proofed before
placing into the body.
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Biomaterials must be compatible with the body, and there are often issues
of biocompatibility which must be resolved before a product can be placed on the
market and used in a clinical setting. Because of this, biomaterials are usually
subjected to the same requirements as those undergone by new drug therapies.
All manufacturing companies are also required to ensure traceability of all of
their products so that if a defective product is discovered, others in the same
batch may be traced.
Biocompatibility
The ability of a material to perform with an appropriate host response in a
specific application. The ability of a biomaterial to perform its desired function
with respect to a medical therapy, without eliciting any undesirable local or
systemic effects in the recipient or beneficiary of that therapy, but generating the
most appropriate beneficial cellular or tissue response to that specific situation,
and optimizing the clinically relevant performance of that therapy.
Host Response: The response of the host organism (local and systemic) to the
implanted material or device.
E.g.: A hemodialysis system serving as an artificial kidney requires materials
that must function in contact with the patients`s blood and exhibit appropriate
membrane permeability and mass transport characteristics.
A long list of ‗non-properties‘ had evolved for ‗successful‘ biomaterials:
nontoxic, non-immunogenic, non-thrombogenic, non-carcinogenic, and so forth.
The above definition required that materials not only provide some function, but
also recognized that the interface created by introduction of the material will
elicit a biological response. Thus, the idea that the material could be truly inert
was essentially rejected with the adoption of this definition. Given today‘s level
of
understanding
of
our
bodies
as
sophisticated,
complex
biological
environments, the idea that one could place a foreign material without some sort
of response seems naive.
The biomaterials must not degrade in its properties within the body and must
not cause any adverse reaction within the host´s body.
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Selection criteria for Biomaterials
Biomaterials and biomedical devices are used throughout the human body:
2 important aspects:
– Functional performance
– Biocompatibility
Functional performance
The material must satisfy its design requirements in service:
Functional performance
Examples
Load transmission and stress
(e.g. bone replacement)
distribution
(e.g. artificial knee
Articulation to allow movement
joint)
Control of blood and fluid flow
(e.g. artificial heart)
Space filling
(e.g. cosmetic surgery)
Electrical stimuli
(e.g. pacemaker)
Light transmission
(e.g. implanted lenses)
Sound transmission
(e.g. cochlear implant)
Selection criteria for Biomaterials
The technical materials used to build most structures are divided into four classes:
– Metals
– Ceramics (including glasses)
– Composites
– Polymers
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SUBJECTS OF BIOMATERIALS SCIENCE
There are a number of important subjects in biomaterials science and
engineering that students learn in the present course. Some of those subjects are
given below:
Toxicology:
Biomaterials should not be toxic. Unless otherwise specified, toxicology is a
study of adverse effects of biomaterials on the living cells and organs of the body.
It usually deals with symptoms, mechanisms, treatment and detection. However,
there are some biomaterials or drugs specifically designed to be toxic to target
deadly diseases (e.g., cancer tumors) and destroy them.
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Biocompatibility:
The biomaterials must be non tumorigenic, normal wound healing, no infections,
no hypersensitivity. Biocompatibility is mostly related to the behavior of
biomaterials inthe body conditions. It is difficult to measure directly, so generally
defined in terms of success for specific applications, such as implants and drug
delivery systems.
Biodegradability:
It is simply a phenomenon that natural and synthetic biomaterials are capable of
decomposing in the body conditions without leaving any harmful substances
behind. Sometimes, it leaves behind useful nutrients, which may be useful for
disease treatment and body recovery.
Targeted Drug Delivery:
Drug targeting is achieved through venous injection of drug loaded materials,
which freely circulate throughout the body. Under the external forces or effects
(e.g., magnetic, ultrasound, electric, temperature, light, X-Rey, pH and
mechanical), these materials are trapped and concentrated at the local site, and
then start releasing the drug molecules. Three main mechanisms for releasing
drug molecules from the materials into a blood vessel or tissue are diffusion,
degradation, and swelling followed by diffusion
Healing:
One of the main considerations of biomaterials is that when they are placed in
the body, they should heal the disfunctioning part of the body.
Mechanical Durability:
The best materials for medical applications are not only biocompatible, but also
have better physical properties similar to those of the bones, tissues or other
biological systems to be replaced or repaired. Biomaterials must perform to
certain standards, and also cope with tensile and compression stresses. Some of
the comparative properties of natural and synthetic biomaterials are given in
Table 1. As is seen, every material has its own special Young‘s modulus, density
and compression and tension strength, which in turn determine their specific
applications in different biomedical purposes.Thus, it is essential that all
biomaterials are well designed and are tested before the medical applications.
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Biomaterials Corrosion:
Body fluid has all kinds of anions (Cl-, HPO42- and HCO3), cations (Na+, K+,
Ca2+, and Mg2+), organic substances (proteins and enzymes), plasma, water and
dissolved oxygen along with body temperature (37°C). Thus, body has all
possible environments for metallic biomaterials corrosion [1]. Figure 4 shows the
corrosion formation on a hip joint at the junction between the modular head and
neck of prosthesis.
Failure of Biomaterials:
Although several biomaterials meet the requirements of biocompatibility for
medical use, unfortunately, some of the biomaterials do not possess sufficient
mechanical durability in a large number of cases. Thus, revision surgeries are
necessary in approximately 7% of hip and 10% of knee replacements after 10
years of use.
Biomaterials can fail through several ways:
(i)
insufficient
mechanical
durability,
higher
fatigue,
damage
accumulation, and wear,
(ii)
provoking adverse biological responses, such as failure arises by bone
loss or bone death due to the inappropriate stressing of the
peri-prosthetic tissues, failure of bone ingrowth due to the relative
motion between implant and tissues or osteolysis due to the wear
particles
Metallic corrosion on a hip joint (left) and degraded polymeric knee joint
(below) shows the degraded polymeric knee joint after a long period of use
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Mechanical and performance requirements

strong and rigid:

soft and elastomeric: Articular cartilage substitute:

strong and flexible:
Hip prothesis:
Dialysis membrane:
Biomaterials can be in the forms of metals and alloys, ceramics, polymers and
composites.
Metals and alloys
Metals and alloys are used as biomaterials due to their excellent mechanical,
surface and thermal properties. Some of the metals and alloys include 316L
stainless steel, Ti based alloys, Cr based alloys, Ni based alloys, Au, Ag and Pt
based metals and alloys, and amalgams (Hg, Ag and Sn). The properties of
metallic materials are related to the grain size and shape, surface roughness and
imperfections in the crystal structure. However, some studies showed that the
surface of metals can be active and interact with the tissue or organs and
produce toxic corrosion products. This limits the use of metallic materials in
various applications.
Ceramic biomaterials
bioceramics are highly biocompatible materials and possess several superior
properties:

they can have structural functions as joint or tissue replacements,

can be used as coatings to improve the biocompatibility of the implants,

can allow growing cells and tissues on them

can be used to replace some of the entire body parts.
The better chemical and thermal stability, strength, wear resistance and
durability make ceramics good candidate materials for surgical implants. The
main disadvantages of the ceramics are that they are highly brittle, have low
tensile strength, can mechanically fail during the use and are not reparable when
broken. Some of the ceramic biomaterials include hydroxyapatite, alumina,
zirconia, calcium phosphate, insoluble glasses, bioactive glasses, porcelain and
carbons. Bioceramics have higher porosity in their structure which is a critical
parameter for growth and integration of cells and tissues.
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The several biomaterials utilized for various biomedical applications.
Polymeric biomaterials
Polymeric biomaterials : possess a wide spectrum of physical, chemical,
physicochemical and biological properties that allow them to be used in a wide
verity of medical applications. They can be both biocompatible and
biodegradable depending on the chemical structures and applications. Some of
the biocompatible polymers include (but are not limited to) ultra-high molecular
weight
polyethylene,
polymethyl-methacrylate,
polytetrafluoroethylene,
polyethleneterephthalate,
poly(etheretherketone),
polyvinyl
chloride,
polyethylene, polypropylene, etc. Biodegradable polymers include polylactide
(PLA), polyglycolide (PGA), polycaprolactone (PCL), and their copolymers. It is
reported that the degradation of the materials yields the corresponding hydroxy
acids, making them safe for in vivo use.
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Ductility, low tensile and compression strengths, and high wear rate result in a
high generation of wear debris, which reduce the applications of some of the
polymeric biomaterials.
Composite biomaterials
Composite biomaterials are new classes of materials formed by a biocompatible
matrix (resin) and a reinforcement of synthetic materials (e.g., carbon and glass
fibers). There are also natural composite biomaterials including bone, wood,
dentin, cartilage, turtle shell, chicken feather, and skin. These materials are used
for drug, gene and DNA delivery, tissue engineering, joint and bone replacement,
cosmetic, orthodontics (dental), etc. These materials usually imitate the
structures of the living parts of the body.
Various micro structures of biomaterials for cell and tissue growth.
Metals
Metals are widely used as biomaterials due to their strength and toughness
Implant metals are generally biocompatible:
– Stainless steel
– Titanium
– Cobalt alloys
Some people are allergic to ions released from these metals.
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Metals used in medicine
Stainless steel (most common 316L)
60-65% iron, 17-19% cromium, 12-14% nickel, > than 0.030% carbon, minor
amounts of nitrogen, manganese, molybdenum, phosphorous, silicon, and sulfur.
Low carbon content for better resistance to in vivo corrosion.
Cromium: corrosion resistance by formation of surface oxide.
Nickel: improves strength by increasing face centered cubic phase (austenite).
Due to potential long term release of Ni2+, Cr3+ and Cr6+ restricted to
temporary devices.
Used as screws and fittings for orthopedics.
Ti alloys
Light , Good mechanical properties , Good corrosion resistance due to TiO2
solid oxid layer.
Pure Ti, grade 4 CP Ti (ASTM F67)
– Ti 99% wt (non-alloyed)
– 0.4% O
– Commercially pure Ti used in dental implants.
– Ti-6Al-4V ELI (ASTM F136)
Ti 89% wt Al 6% wt V 4%
is widely used for implants and porous surface-coating.
– Contains impurities (N, 0.13% O, Fe, H, C)
– Impurities increasing interstitial content increase strength and fatigue limit.
(N increases hardening, O increases tensile (yield) strength)
Ti-6Al-4V ELI (ASTM F136)
• Hip and knee implants
• Screws and fittings
• Dental implants
• Pacemaker housings
– Resistant to stress corrosion cracking and corrosion fatique in body fluids.
– One of the few materials that permits bone growth at the interface. But,
titanium has unsatisfactory wear
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Dental Metals
Amalgam
– Used since 150 years
– Mixture of solid alloy and mercury (moldable and strong).
– Solid alloy composed of silver (65%), tin (29%), copper (6%), zinc (2%) and
mercury.
• Mercury (3%) is fluid in mixture and forms Ag3Sn, Ag2Hg3, Sn7Hg
– Deformable mixture packed in cavety
– Hardens in time (25% of total strength in 1 hour, full strenght in a day).
– Amalgam is stable, strong, inexpensive and endurable (8-10years).
Gold and gold alloy:
Noble metal-known since thousands of years
– First dental biomaterial
– Biocompatible
– Pure gold is to soft for dental applications
– Enrichment by platinum
– Excellent biomaterial/processing/dental applications
– Resist high mechanical loading
– High endurance
– Aesthetics: metallic color
Alloys
–1400 different dental alloys
– Applicable for all kind of restoration
– Contain only biocompatible materials
– Palladium, Silver, Gold, Cupper, Platinum
– Should not contain: Ni, Cd, Be
Corrosion
It is continued degradation of metals to oxide, hydroxide or other compounds
through chemical reactions.
The human body is an aggressive medium for inducing corrosion in metals:
water, dissolved oxygen, proteins, chloride and hydroxide
Corrosion: Basic Reactions
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Ionization: Direct formation of metalic cations under acidic or reducing (i.e.
oxygen poor) conditions.
M → M+ + eOxidation: Direct reaction of metal with oxygen. M + O2 → MO2
Hydroxylation: The reaction of water under alkaline (basic) or oxidizing
conditions to yield a hydroxide or hydrated oxide.
2M + O2(aq) + 2H2O → 2M(OH)2
Corrosion Mechanism:
– Materials have tendency to reach their lowest possible free energy (corroded
state is preferred).
– Most alloys, oxides, hydroxides, sulfides have negative free energy of formation
and they are thermodynamically favored over the pure metal
– Metal atoms ionize, go into solution and combine with oxygen.
– Metal flakes off
To avoid corrosion:
Consider the composition of the biological invironment (ions, pH, oxygen
pressure, etc.)
– Use appropriate metals.
– Avoid implantation of dissimilar metals.
– Minimize pits and crevices.
– Avoid transfer of metal from tools to the implant during surgery.
Ceramics, Glasses, and Glass-Ceramics
Ceramics,
Glasses,
and
Glass-Ceramics
include
a
broad
range
of
inorganic/nonmetallic compositions.
– Eyeglasses
– Diagnostic instruments
– Thermometers
– Tissue culture flasks
– Fiber optic for endoscopy
Dentistry (gold-porcelain crowns, glass-filled ionomer cements, and dentures)
Refractory compounds/materials and Usually some combination of metal and
nonmetal in general AmXn structural form (A = metal; X = nonmetal)
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Relative size of ions (radius ratio) and degree of covalent/ionic bonding
determine atomic arrangements.
High oxidized state and ion/covalent bonding in ceramics makes them:
– Resistant to oxidation and increases stability
– Non conducting
– High melting temps
– Hard and brittle
Generally used to repair or replace skeletal hard connective tissue. No one
material is suitable for all biomaterial applications.
– Their success depend upon achieving a stable attachment to connective tissue.
– Tissue attachment is directly related to the type of tissue response at the
implant-tissue interface.
– No material implanted is inert; all materials elicited a response from the tissue.
Types of tissue response:
Bioceramics
Ceramics are: Stiff , Hard, Chemically stable, Wear resistant
Material properties differ greatly dependent on (thermal) processing method,
yielding
5 categories of microstructure:
– Glass
– Cast or plasma-sprayed polycrystalline ceramic
– Liquid-sintered (vitrified) ceramic
– Solid-state sintered ceramic
– Polycrystalline glass-ceramic
Of the large number of ceramics known only a few are suitable biocompatible.
Main problems: They are brittle, Relatively difficult to process
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Bioinert Bioceramics: Alumina (Al2O3)
High density , high purity (>99.5%) alumina
Very chemically inert
Excellent corrosion resistance
High wear resistance, but Low fracture thoughness
And tensile strenght (high elastic moculus)
Used in compression only (to reduce encapsulation thickness)
Femoral head of total hip replacements
Orthopedic implants in general
Dental implants
Dental Ceramics
Excellent aesthetics (opaqueness & color)
Very tough and hard material But brittle; improvements of
strength necessary (achieved by proper processing)
Expensive manufacture (dental labs)
Alteration of opaqueness & color possible
Polymers
Carbon atoms are usually joined in a linear chainlike structure and substituted
with a great variety of atoms, molecules or functional groups. Thermoplastic
polymers: Basic chains with little or no branching; can be melted and remelted
without a basic change in structure (=crystalline) .Thermosetting polymers: Side
chains form covalent links between chains (threedimensional network); do not
melt uniformly on reheating. (= chemically cross-linked)
Polymers
Non-degradable
Biodegradable
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Polymer-classes used in medicine
Biodegradation:
A biological agent (an enzyme, or cell) is responsible for degradation.
Bioerosion:
contains both physical (such as dissolution) and chemical processes (such as
backbone cleavage). E.g. a water-insoluble polymer that turns water-soluble
under physiological conditions.
Bioresorption, Bioabsorption:
Polymer or its degradation products removed by cellular activity
(e.g.Phagocytosis).
Natural Materials
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Fabrics
Major biomedical applications:
Surgical gowns: mostly woven and non-woven cellulose, polyethylene and poly
propylen fibers. Masks and shoe covers: made of gauze and nonwoven fabrics,
respectively.
Adhesive tapes: woven or knitted fabric strip with adhesive film.
Wound repair and reconstruction of soft tissue.
Sutures and threads used to close wounds
Ligation threads to tie off bleeding vessels
Fiber or fabric reinforced
implants for reconstructive and repair surgery of soft tissues.
Cardiovascular system applications:
Vascular grafts made of woven, knitted or micro porous constructions
Dacron, Teflon, nylon66 and polypropylene have been used in
prosthetic heart valves as „sewing ring―.
Musculoskeletal system applications:
Artificial tendons and ligaments
Matrices for reconstructive and maxillofacial surgery
Graphite-Teflon fibers meshes as matrices for tissue
ingrowth in stabilization of dental or orthopedic implants
Percutaneous and cutaneous application:
Shunts: to provide access to the circulation for routine dialysis
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Biologically Functional Materials
Enzymes, antibodies, drugs, or cells have been immobilized on and with
polymeric systems for a wide range of therapeutic, diagnostic and bioprocess
applications .
Host reactions to biomaterials
Complications
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Complications are largely based on biomaterial-tissue interactions that include
both:
Effects of the implant on the host tissue and effects of the host on the implant.
Inflammation Foreign body reaction (FBR), Immunological response, Systemic
toxicity, Blood-surface interactions, Thrombosis, Device-related infections,
Tumorigenesis.
Based on the reaction of the tissue to the biomaterial, these are classified into
three distinct categories:
1. Biotolerant Materials: which are separated from bone tissue by a layer of
fibrous tissue.
2. Bioactive materials: which have the property of establishing chemical bonds
with bone tissue, known as osseointegration. The collagen and mineral phase of
the adjacent bone is deposited directly on the implant surface.
3. Bioinert Materials: in this class it is possible, under certain conditions, to have
direct contact with the adjacent bone tissue. No chemical reactions shall occur
between the implant and the tissue.
Recognition of an active interface between biomaterials and biological systems
led to several important basic ideas about biocompatibility. These ideas persist
today and comprise the essence of biocompatibility.
The first idea is that the interactions at the material–tissue interface occur for
both; the material elicits a response from the body and the body elicits a
response from the material. All materials will be changed at some level by their
introduction into a biological environment—either via corrosion, chemical
modification, deposition of substance, degradation, or other mechanism. This
exchange of responses leads to a second idea: that the material–tissue interface is
dynamic. As the material and biological tissue are modified by each other, the
changes themselves may suppose other changes. Thus, the interface is not static,
but is changing over its lifetime. Furthermore, because the human buccal
conditions are always changing—by aging, by developing systemic or local
diseases by adopting new activities, by eating differently, etc.—any equilibrium
established at a material–tissue interface is subject to perturbation. A third idea
is that reactions at the material–tissue interface are a function of the tissue where
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the interface is created. A fourth idea about biological–tissue interfaces
recognizes the nearly obvious, but often forgotten fact that the materials we use
do not belong there. Biomaterials are foreign bodies, and biological responses to
these materials are characterized by foreign body responses. Finally, the most
recent idea about biocompatibility is that it is possible to customize interactions
at the material–tissue interface. Materials are asked to play more sophisticated,
longer-term roles in tissues, customizing and optimizing the material–tissue
interface to assure the best long term clinical outcomes.
The Atom :
What is your body made of ? Your first thought might be that it is made up of
different organs, such as your heart, lungs, and stomach, that work together to
keep your body going. Or you might zoom in a level and say that your body is
made up of many different types of cells. However, at the most basic level, your
body (and, in fact, all of life, as well as the non-living world) is made up of atoms,
often organized into larger structures called molecules.
Atoms and molecules, despite being part of a complex, living, breathing being,
still follow the rules of chemistry and physics. If you learned in chemistry that
some atoms tend to gain or lose electrons, or form bonds with each other, those
facts remain true even when the atoms or molecules are part of a living thing. In
fact, simple interactions between atoms – played out many times, and in many
different combinations, in a single cell or a larger organism – are what makes life
possible. One could argue that everything you are, including your consciousness,
is the byproduct of chemical and electrical interactions between a very, very
large number of non-living atoms!
Matter and elements
The term matter just refers to anything that occupies space and has mass - in
other words, the ―stuff‖ that the universe is made of. All matter is made up of
substances called elements, which have specific chemical and physical properties
and cannot be broken down into other substances through ordinary chemical
reactions. Gold, for instance, is an element, and so is carbon. There are 118
elements, but only 92 occur naturally. The remaining elements have only been
made in laboratories and are unstable.
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Each element is designated by its chemical symbol, which is a single capital letter
or, when the first letter is already ―taken‖ by another element, a combination of
two letters. Some elements follow the English term for the element, such as C for
carbon and Ca for calcium. Other elements‘ chemical symbols come from their
Latin names; for example, the symbol for sodium is Na, which is a short form
of natrium, the Latin word for sodium.
The four elements common to all living organisms are oxygen (O), carbon (C),
hydrogen (H), and nitrogen (N), which together make up about 96% of the
human body. In the non-living world, elements are found in different
proportions, and some elements common to living organisms are relatively rare
on the earth as a whole. All elements and the chemical reactions between them
obey the same chemical and physical laws, regardless of whether they are a part
of the living or non-living world.
The structure of the atom
An atom is the smallest unit of matter that retains all of the chemical properties
of an element. For example, a gold coin is simply a very large number of gold
atoms molded into the shape of a coin (with small amounts of other,
contaminating elements). Gold atoms cannot be broken down into anything
smaller while still retaining the properties of gold. A gold atom gets its properties
from the tiny subatomic particles it's made up of.
An atom consists of two regions. The first is the tiny atomic nucleus, which is in
the
center
of
the
atom
and
contains
positively
charged
particles
called protons and neutral (uncharged) particles called neutrons. The second,
much larger, region of the atom is a ―cloud‖ of electrons, negatively charged
particles that orbit around the nucleus. The attraction between the positively
charged protons and negatively charged electrons holds the atom together. Most
atoms contain all three of these types of subatomic particles (protons, electrons,
and neutrons), although hydrogen (H) is an exception because it typically has one
proton and one electron, but no neutrons. The number of protons in the nucleus
determines what element an atom is, while the number of electrons surrounding
the nucleus determines what kind of reactions the atom will undergo. The three
types of subatomic particles are illustrated below for an atom of helium (which,
by definition, contains two protons).
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Structure of an atom. The protons (+ charge) and neutrons (neutral charge) are
found together in the tiny nucleus at the center of the atom. The electrons
(- charge) occupy a large, spherical cloud surrounding the nucleus. The atom
shown is this particular image is helium, with two protons, two neutrons, and two
electrons.
Protons and neutrons do not have the same charge, but they do have
approximately the same mass, about 1.67 × 10-24. Since grams are not a very
convenient unit for measuring masses that tiny, scientists chose to define an
alternative measure, the dalton or atomic mass unit (amu). A single neutron or
proton has a weight very close to 1 amu.
Electrons are much smaller in mass than protons, only about 1/1800 of an atomic
mass unit, so they do not contribute much to an element‘s overall atomic mass.
On the other hand, electrons do greatly affect the atom‘s charge, as each electron
has a negative charge equal to the positive charge of a proton. In uncharged,
neutral atoms, the number of electrons orbiting the nucleus is equal to the
number of protons inside the nucleus. The positive and negative charges cancel
out, leading to an atom with no net charge.
Protons, neutrons, and electrons are very small, and most of the volume of an
atom—greater than 99 percent—is actually empty space. With all this empty
space, you might ask why so-called solid objects don‘t just pass through one
another. The answer is that the negatively charged electron clouds of the atoms
will repel each other if they get too close together, resulting in our perception of
solidity
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The nucleus
The nucleus is at the center of the atom and contains the protons and neutrons.
Protons and neutrons are collectively known as nucleons.
Virtually all the mass of the atom is concentrated in the nucleus, because the
electrons weigh so little.
Working out the numbers of protons and neutrons
No of protons = ATOMIC NUMBER of the atom
The atomic number is also given the more descriptive name of proton number.
No of protons + no of neutrons = MASS NUMBER of the atom
The mass number is also called the nucleon number.
This information can be given simply in the form:
How many protons and neutrons has this atom got?
The atomic number counts the number of protons (9); the mass number counts
protons + neutrons (19). If there are 9 protons, there must be 10 neutrons for the
total to add up to 19.
The atomic number is tied to the position of the element in the Periodic Table
and therefore the number of protons defines what sort of element you are talking
about. So if an atom has 8 protons (atomic number = 8), it must be oxygen. If an
atom has 12 protons (atomic number = 12), it must be magnesium.
Similarly, every chlorine atom (atomic number = 17) has 17 protons; every
uranium atom (atomic number = 92) has 92 protons.
Isotopes
The number of neutrons in an atom can vary within small limits. For example,
there are three kinds of carbon atom 12C, 13C and14C. They all have the same
number of protons, but the number of neutrons varies.
26
carbon-12
carbon-13
carbon-14
protons
6
6
6
neutrons
6
7
8
mass number
12
13
14
These different atoms of carbon are called isotopes. The fact that they have
varying numbers of neutrons makes no difference whatsoever to the chemical
reactions of the carbon.
Isotopes are atoms which have the same atomic number but different mass
numbers. They have the same number of protons but different numbers of
neutrons.
The electrons
Working out the number of electrons
Atoms are electrically neutral, and the positiveness of the protons is balanced by
the negativeness of the electrons. It follows that in a neutral atom:
No of electrons = No of protons
So, if an oxygen atom (atomic number = 8) has 8 protons, it must also have 8
electrons; if a chlorine atom (atomic number = 17) has 17 protons, it must also
have 17 electrons.
The arrangement of the electrons
The electrons are found at considerable distances from the nucleus in a series of
levels called energy levels. Each energy level can only hold a certain number of
electrons. The first level (nearest the nucleus) will only hold 2 electrons, the
second holds 8, and the third also seems to be full when it has 8 electrons.
e.g. to find the electronic arrangement in chlorine

The Periodic Table gives you the atomic number of 17.

Therefore there are 17 protons and 17 electrons.

The arrangement of the electrons will be 2, 8, 7 (i.e. 2 in the first level, 8 in
the second, and 7 in the third).
Main types of chemical bonds:
27
Materials science encompasses various classes of materials; materials are
sometimes classified by the type of bonding present between the atoms :
Ionic bonding
Covalent bonding
Metallic bonding
A chemical bond is an attraction between atoms. This attraction may be seen as
the result of different behaviors of the outermost or valence electrons of atoms.
Although all of these behaviors merge into each other seamlessly in various
bonding situations so that there is no clear line to be drawn between them, the
behaviors of atoms become so qualitatively different as the character of the bond
changes quantitatively, that it remains useful and customary to differentiate
between the bonds that cause these different properties of condensed matter.
In the simplest view of a so-called 'covalent' bond, one or more electrons (often a
pair of electrons) are drawn into the space between the two atomic nuclei. Here
the negatively charged electrons are attracted to the positive charges of both
nuclei, instead of just their own. This overcomes the repulsion between the two
positively charged nuclei of the two atoms, and so this overwhelming attraction
holds the two nuclei in a fixed configuration of equilibrium, even though they
will still vibrate at equilibrium position. Thus, covalent bonding involves sharing
of electrons in which the positively charged nuclei of two or more atoms
simultaneously attract the negatively charged electrons that are being shared
between them. These bonds exist between two particular identifiable atoms and
have a direction in space, allowing them to be shown as single connecting lines
between atoms in drawings, or modeled as sticks between spheres in models.
In a polar covalent bond, one or more electrons are unequally shared between
two nuclei. Covalent bonds often result in the formation of small collections of
better-connected atoms called molecules, which in solids and liquids are bound
to other molecules by forces that are often much weaker than the covalent bonds
that hold the molecules internally together. Such weak intermolecular bonds give
28
organic molecular substances, such as waxes and oils, their soft bulk character,
and their low melting points (in liquids, molecules must cease most structured or
oriented contact with each other). When covalent bonds link long chains of
atoms in large molecules, however (as in polymers such as nylon), or when
covalent bonds extend in networks through solids that are not composed of
discrete molecules (such as diamond or quartz or the silicate minerals in many
types of rock) then the structures that result may be both strong and tough, at
least in the direction oriented correctly with networks of covalent bonds. Also,
the melting points of such covalent polymers and networks increase greatly.
In a simplified view of an ionic bond, the bonding electron is not shared at all,
but transferred. In this type of bond, the outer atomic orbital of one atom has a
vacancy which allows the addition of one or more electrons. These newly added
electrons potentially occupy a lower energy-state (effectively closer to more
nuclear charge) than they experience in a different atom. Thus, one nucleus
offers a more tightly bound position to an electron than does another nucleus,
with the result that one atom may transfer an electron to the other. This transfer
causes one atom to assume a net positive charge, and the other to assume a net
negative charge. The bond then results from electrostatic attraction between
atoms and the atoms become positive or negatively charged ions. Ionic bonds
may be seen as extreme examples of polarization in covalent bonds. Often, such
bonds have no particular orientation in space, since they result from equal
electrostatic attraction of each ion to all ions around them. Ionic bonds are
strong (and thus ionic substances require high temperatures to melt) but also
brittle, since the forces between ions are short-range and do not easily bridge
cracks and fractures. This type of bond gives rise to the physical characteristics
of crystals of classic mineral salts, such as table salt.
A less often mentioned type of bonding is metallic bonding. In this type of
bonding, each atom in a metal donates one or more electrons to a "sea" of
electrons that reside between many metal atoms. In this sea, each electron is free
(by virtue of its wave nature) to be associated with great many atoms at once.
The bond results because the metal atoms become somewhat positively charged
due to loss of their electrons while the electrons remain attracted to many atoms,
29
without being part of any given atom. Metallic bonding may be seen as an
extreme example of delocalization of electrons over a large system of covalent
bonds, in which every atom participates. This type of bonding is often very
strong (resulting in the tensile strength of metals). However, metallic bonding is
more collective in nature than other types, and so they allow metal crystals to
more easily deform, because they are composed of atoms attracted to each other,
but not in any particularly-oriented ways. This results in the malleability of
metals. The sea of electrons in metallic bonding causes the characteristically
good electrical and thermal conductivity of metals, and also their "shiny"
reflection of most frequencies of white light.
Ionic bonding is a type of electrostatic interaction between atoms which have a
large electronegativity difference. There is no precise value that distinguishes
ionic from covalent bonding, but a difference of electronegativity of over 1.7 is
likely to be ionic, and a difference of less than 1.7 is likely to be covalent.[5] Ionic
bonding leads to separate positive and negative ions. Ionic charges are commonly
between −3e to +3e. Ionic bonding commonly occurs in metal salts such as
sodium chloride (table salt). A typical feature of ionic bonds is that the species
form into ionic crystals, in which no ion is specifically paired with any single
other ion, in a specific directional bond. Rather, each species of ion is
surrounded by ions of the opposite charge, and the spacing between it and each
of the oppositely charged ions near it, is the same for all surrounding atoms of
the same type. It is thus no longer possible to associate an ion with any specific
other single ionized atom near it. This is a situation unlike that in covalent
crystals, where covalent bonds between specific atoms are still discernible from
the shorter distances between them, as measured via such techniques as X-ray
diffraction.
Ionic crystals may contain a mixture of covalent and ionic species, as for example
salts of complex acids, such as sodium cyanide, NaCN. Many minerals are of this
type. X-ray diffraction shows that in NaCN, for example, the bonds between
sodium cations (Na+) and the cyanide anions (CN−) are ionic, with no sodium ion
associated with any particular cyanide. However, the bonds between C and N
atoms in cyanide are of the covalent type, making each of the carbon and
31
nitrogen associated with just one of its opposite type, to which it is physically
much closer than it is to other carbons or nitrogens in a sodium cyanide crystal.
When such crystals are melted into liquids, the ionic bonds are broken first
because they are non-directional and allow the charged species to move freely.
Similarly, when such salts dissolve into water, the ionic bonds are typically
broken by the interaction with water, but the covalent bonds continue to hold.
For example, in solution, the cyanide ions, still bound together as single CN−
ions, move independently through the solution, as do sodium ions, as Na+. In
water, charged ions move apart because each of them are more strongly
attracted to a number of water molecules, than to each other. The attraction
between ions and water molecules in such solutions is due to a type of weak
dipole-dipole type chemical bond. In melted ionic compounds, the ions continue
to be attracted to each other, but not in any ordered or crystalline way.
Covalent bonding is a common type of bonding, in which the electronegativity
difference between the bonded atoms is small or nonexistent. Bonds within most
organic compounds are described as covalent. See sigma bonds and pi bonds for
LCAO-description of such bonding.
A polar covalent bond is a covalent bond with a significant ionic character. This
means that the electrons are closer to one of the atoms than the other, creating
an imbalance of charge. They occur as a bond between two atoms with
moderately
different
electronegativities
and
give
rise
to
dipole-dipole
interactions. The electronegativity of these bonds is 0.3 to 1.7.
A coordinate covalent bond is one where both bonding electrons are from one of
the atoms involved in the bond. These bonds give rise to Lewis acids and bases.
The electrons are shared roughly equally between the atoms in contrast to ionic
bonding. Such bonding occurs in molecules such as the ammonium ion (NH4+)
and are shown by an arrow pointing to the Lewis acid. Also known as non-polar
covalent bond, the electronegativity of these bonds range from 0 to 0.3.
31
Molecules which are formed primarily from non-polar covalent bonds are often
immiscible in water or other polar solvents, but much more soluble in non-polar
solvents such as hexane.
In metallic bonding, bonding electrons are delocalized over a lattice of atoms. By
contrast, in ionic compounds, the locations of the binding electrons and their
charges are static. The freely-moving or delocalization of bonding electrons leads
to classical metallic properties such as luster (surface light reflectivity), electrical
and thermal conductivity, ductility, and high tensile strength.
Elements, compounds and chemical equations
All matter is made of atoms. There are over a hundred different types of atom,
called elements, and each one has a symbol. The atoms of a particular element
are identical to each other.
When atoms of different elements join together they make a compound.
Compounds are represented by formulae that show how many atoms of each
element are in the compound. Formulae can be worked out from the charge of
the ions that make up the compound.
Chemical reactions can be written down as balanced chemical equations.
Elements and compounds
All matter is made from atoms. Atoms are very, very small. A molecule is formed
when atoms join together by chemical bonds.
There are over a hundred different types of atom, called elements. The atoms of
a particular element are identical to each other. They cannot be changed
chemically into any different element. For example a piece of pure sulfur consists
only of sulfur atoms. No chemical process can break it down into a different
substance.
Compounds are substances that contain atoms of at least two elements
chemically combined. For example: magnesium oxide consists of magnesium
atoms and oxygen atoms chemically bonded together.
Examples of compounds and elements
32
Substance
Types of atom chemically combined
So it is...
hydrogen
hydrogen
an element
sodium chloride
sodium and chlorine
a compound
carbon
carbon
an element
carbon dioxide
carbon and oxygen
a compound
Notice how the name sometimes suggests whether a substance is an element or a
compound. If you can see the names of two or more elements it's almost certainly
a compound but beware – many substances with only one name are actually
compounds – water and methane for example.
Symbols and formulae
Elements are represented by one or two letters. The first letter is always a capital
and the second is always lower case.
Formulae
Compounds are represented by formulae. Symbols and numbers show
Element
Symbol
Oxygen
O
Sodium
Na
Carbon
C
Zinc
Zn
the atoms in the compound. For example: ZnCO3 is the formula of zinc
carbonate. One zinc atom (Zn) and one carbon atom (C) are chemically
bonded with three oxygen atoms (O3). Notice that we don't need to write
a 1 next to the Zn or C.
Equations
When the atoms in elements or compounds rearrange themselves to form new
substances it's called a chemical reaction. The atoms are rearranged but no
atoms are lost or made. Chemical reactions are written down as chemical
equations.
All substances are made from atoms. Each atom is made of a nucleus containing protons and neutrons - surrounded by electrons.
The atomic number is the number of protons in an atom. The elements are
arranged in the Periodic Table in ascending order of atomic number.
33
The mass number of an atom is the total of protons plus neutrons. Atoms of the
same element with different numbers of neutrons (and hence different mass
numbers) are called isotopes of that element.
Atomic structure
All material things are made from atoms. There are just over one hundred
different types of atom, called elements. Atoms can join together in millions of
different combinations to make all the substances on Earth and beyond.
Structure of the atom
Every atom is made of a nucleus consisting of protons and neutrons. The nucleus
is surrounded by electrons. Protons and electrons are oppositely charged.
Neutrons have no charge. This means the nucleus of an atom is always positively
charged. An atom has a neutral overall charge because it has the same number
of electrons as protons. Protons and neutrons have the same mass. Electrons
have such a small mass that this can usually be taken as zero.
Comparing the charge and mass of electrons, protons and neutrons
Proton
Neutron
Electron
Charge
+1
0
-1
Mass
1
1
0.0005 (almost zero)
The atomic number (also called the proton number) is the number of protons in
an atom.
The mass number (also called the nucleon number) is the total number of
protons and neutrons in an atom.
34
The elements are arranged in the Periodic Table in ascending order of atomic
number so it's easy to find the name or symbol for an atom if you know the
atomic number.
Section of the periodic table showing elements ascending by atomic number
This zinc ion has a mass number of 65, an atomic number of 30
The number of protons in an atom is the same as the atomic number.
The number of neutrons is the mass number minus the atomic number.
Ionic bonding occurs between positive and negative ions, which attract each
other and bind together to form ionic compounds. For example, sodium chloride
consists of Na+ ions and Cl- ions bound together.
Each ion is surrounded by oppositely charged ions held in place by electrostatic
attraction and forming an ionic crystal lattice. The ions in a crystal lattice are
very strongly bonded - a high temperature is required to melt the crystal.
There are several ways in which atoms chemically combine together to make
compounds. One of these ways is called ionic bonding. Atoms turn into ions when
they lose or gain electrons.
Metal ions
In some circumstances metal atoms may lose electrons. The atom then has more
protons than electrons and so it will be positively charged, a positive ion.
Example: A magnesium atom may lose two electrons and become a Mg2+ ion.
Non-metal ions
Non-metal atoms may gain electrons and become negatively charged.
Example: An oxygen atom may gain two electrons and become an O2- ion.
35
Positive and negative ions attract one another and bind together forming a new
substance. This is called ionic bonding.
For example:
Sodium chloride consists of Na+ ions and Cl- ions bound together.
Magnesium oxide consists of Mg2+ ions and O2- ions bound together.
Ionic compounds like magnesium oxide and sodium chloride have high melting
points and do not conduct electricity when solid. They do conduct electricity
when molten. Sodium chloride is soluble in water and the solution conducts
electricity.
The formation of ionic compounds is often shown with dot and cross models.
For example, a sodium atom loses one electron from its outer shell to become a
positively charged sodium ion. It can be represented in a diagram like this:
A sodium atom and a sodium ion
On the other hand an oxygen atom gains two electrons in its outer shell to
become a negatively charged oxygen ion:
36
An oxygen atom and an oxide ion
Notice how atoms gain or lose just the right number of electrons to produce an
ion with a complete outer shell of electrons. This is sometimes called a stable
octet.
The positive and negative ions attract each other and form an ionic bond. We
can draw these using dots for one atom and crosses for the other atom. This is
called a dot and cross diagram.
Ionic lattice
Each ion is surrounded by oppositely charged ions held in place by electrostatic
attraction and forming an ionic crystal lattice.
The number of ions in an ionic compound is such that the overall charge of a
sample of the compound is zero.
37
Example
Magnesium chloride consists of Mg2+ and Cl - ions. Twice as many chloride ions
as magnesium ions are needed to make the substance neutral. The formula of
magnesium chloride is MgCl2.
The ions in a crystal lattice are very strongly bonded together so a high
temperature is required to separate the particles and melt the crystal. The ions
are held strongly in position so they cannot move and carry an electric current.
If an ionic compound is melted or dissolved in water the ions are then free to
move and so the substance can be electrolysed.
A covalent bond is formed between non metal atoms, which combine together by
sharing electrons. Covalent compounds have no free electrons and no ions so
they don't conduct electricity.
The Periodic Table is an arrangement of the elements in order of atomic
number. Elements in the same vertical column are in the same group or family
and have similar chemical properties.
Covalent bonding
Non metals combine together by sharing electrons. The shared pair of electrons
holds the two atoms together. It's called a covalent bond. The group of atoms
bonded together in this way is called a molecule.
The types and numbers of atoms in a molecule are shown in its formula.
Examples of covalent molecules
Name
Structure
Hydrogen (H2)
38
Model
Name
Structure
Model
Water (H2O)
Ammonia (NH3)
Methane (CH4)
Covalent compounds are usually gases or liquids with low melting points or
boiling points and they don't conduct electricity.
Example:
Carbon dioxide is a gas with a boiling point of -44°C. It doesn't conduct
electricity.
Water is a liquid with a melting point of 0°C. It doesn't conduct electricity.
Dot and cross models show how a pair of electrons forms a covalent bond. Notice
that in the diagrams in the table below only the electrons in the outer shell of
each atom are shown.
Examples of dot and cross models
Molecule
Dot and cross model
Hydrogen (H2)
39
Molecule
Dot and cross model
Chlorine (Cl2)
Methane (CH4)
Water (H2O)
Carbon dioxide (CO2)
A molecule of carbon dioxide
Low melting and boiling points
The covalent bonds binding the atoms together are very strong but there are
only very weak forces holding the molecules to each other (the intermolecular
forces). Therefore, only a low temperature is needed to separate the molecules
when they're melted or boiled.
Non-conductors
Covalent compounds have no free electrons and no ions so they don't conduct
electricity.
41
Metal structures and their properties
The physical properties of different metals make them useful for different
purposes. For example, aluminium conducts heat, which makes it a useful
material for making saucepans and gold is shiny, which makes it an attractive
material for jewellery.
Metal particles are held together by strong metallic bonds, which is why they
have high melting and boiling points. The loose electrons in metals can all move
together through the metal – creating an electric current.
Properties of metals
The physical properties of metals
Metal
Property
Use
Aluminium
Heat conductor
Make saucepans
Copper
Electrical conductor
Make electric wiring
Gold
Lustrous (shiny)
Make jewellery
Lead
Dense
To add weight when scuba diving
Platinum
High melting and boiling point
Electrodes of spark plugs
Steel


Tungsten
High tensile strength
Strong


Hard
Make ropes for anchoring an oil rig
Make bridges, buildings and cars
Make drill bits
Metallic bonding
The particles in a metal are held together by strong metallic bonds. It takes a lot
of energy to separate the particles. That is why they have high melting points and
boiling points.
Solid metals are crystalline - the particles are close together and in a regular
arrangement.
41
Metals have loose electrons in the outer shells which form a ‗sea‘ of delocalised
negative charge around the close-packed positive ions. There are strong
electrostatic forces holding the particles together.
Conductivity
Maglev trains are held just above the tracks by powerful superconducting
electromagnets. The loose electrons in metals can all move together through the
metal – an electric current.
At very low temperatures some metals conduct electricity very easily indeed.
They have little or no resistance and so enormous currents can be produced
without using large amounts of energy. They're called superconductors.
In the future, the low resistance of superconductors may allow transmission of
electricity without losing energy as heat on the way. Superconductors may be
used to make super-fast electronic circuits so that computers will work even
faster.
The very high electric currents possible in superconducting wires mean that very
powerful electromagnets can be made. These are used in MRI scanners – a noninvasive system for investigating the workings of the living body.
At present superconductors only work at very low temperatures so they have to
be kept very cold with liquid nitrogen and liquid helium. A lot of work is going
into developing superconductors that will work at normal temperatures.
Properties of metals?
42
The versatility of metals attests to the very wide range of properties of the more
than 70 metals on the periodic table. A description of all of these properties and
the applications in which they are used is well beyond the scope of this
section. The following therefore provides an introduction to some of the more
prominent properties
Chemical
Properties:
Metals combine with other metals and some non-metallic elements to form a vast
number of alloys that enhance the properties of metals in specific applications,
e.g., the combination of iron, nickel and chromium provides a series of stainless
steel alloys that are in common use. Metals such as nickel, vanadium,
molybdenum, cobalt, rare earths and the platinum group metals enable the
catalytic reactions for the synthesis of many organic chemicals from petroleum.
A wide variety of metal compounds and salts impart beneficial properties to
products like plastics in terms of colour, brightness, flame resistance and
resistance to degradation. Photography has been made possible by the effect of
light on metal salts.
Mechanical
Properties:
The properties of strength and ductility enable the extensive use of metals in
structures and machinery. Metals and alloys exhibit ductility, malleability and
the ability to be deformed plastically (that is, without breaking), making them
easy to shape into beams (steel beams for construction), extrusions (aluminum
frames for doors and windows), coins, metal cans and a variety of fasteners (nails
and paper-clips). The strength of metals under pressure (compression),
stretching (tensile) and sheer forces makes them ideal for structural purposes in
buildings, automobiles, aircraft frames, gas pipelines, bridges, cables, and some
sports equipment.
Conductivity:
Metals are excellent conductors of both heat and electricity. In general,
conductivity increases with decreasing temperature, so that, at absolute zero (273°C), conductivity is infinite; in other words, metals become superconductors.
Thermal conductivity is harnessed in automobile radiators and cooking utensils.
Electrical conductivity provides society with the ability to transmit electricity
43
over long distances to provide lights and power in cities remote from electrical
generating stations. The circuitry in household appliances, television sets and
computers relies on electrical conductivity. Resistance to Wear, Corrosion,
Fatigue and Temperature: Metals are hard and durable. They are used in
applications sensitive to corrosion such as chemical plants, food preparation,
medical applications, plumbing and lead in storage batteries. Wear resistance is
critical in bearings for all modes of transportation and in machine tools. Fatigue
resistance - the ability to resist breaking after repeated deformation such as
bending - enables the use of metals in springs, levers and gears. Temperature
resistance makes metals suitable for jet engines and filaments in light-bulbs.
Optical Characteristics: Metals are uniformly lustrous and, except for copper
and gold, are silvery or greyish. This is because all metals absorb light at all
frequencies and immediately radiate it. Metals impart mirrors with their
reflective surface. The lustre of metals gives them the attractive appearance that
is so important in jewellery and coins. (Interestingly, metals also provide the
intangible, distinctive "metallic ring" that is associated with coins.)
Magnetic
Properties:
Ferromagnetism is exhibited by iron and several other metals. In addition, other
metals and alloys can be magnetized in an electrical field to exhibit
paramagnetism. Magnetic properties are employed in electric motors,
generators, and speaker systems for audio equipment. Emission Properties:
Metals emit electrons when exposed to radiation (e.g. light) of a short wavelength
or when heated to sufficiently high temperatures. These phenomena are
exploited in television screens, using rare earth oxides and in a variety of
electronic devices and instruments. Conversely, the ability of metals such as lead
to absorb radiation is employed in shielding, for example in the apron provided
by dentists during an X-ray examination.
Crystal structure
In mineralogy and crystallography, a crystal structure is a unique arrangement
of atoms, ions or molecules in a crystalline liquid or solid.[3] It describes a highly
ordered structure, occurring due to the intrinsic nature of its constituents to
form symmetric patterns.
44
The box can be thought of as an array of 'small boxes' infinitely repeating in all
three spatial directions. Such a unit cell is the smallest unit of volume that
contains all of the structural and symmetry information to build-up the
macroscopic structure of the lattice by translation.
Patterns are located upon the points of a lattice, which is an array of points
repeating periodically in three dimensions. The lengths of the edges of a unit cell
and the angles between them are called the lattice parameters. The symmetry
properties of the crystal are embodied in its space group.
A crystal's structure and symmetry play a role in determining many of its
physical properties, such as cleavage, electronic band structure, and optical
transparency.
Unit cell
The crystal structure of a material (the arrangement of atoms within a given
type of crystal) can be described in terms of its unit cell. The unit cell is a small
box containing one or more atoms arranged in 3-dimensions. The unit cells
stacked in three-dimensional space describe the bulk arrangement of atoms of
the crystal. The unit cell is represented in terms of its lattice parameters, which
are the lengths of the cell edges (a,b and c) and the angles between them (alpha,
beta and gamma), while the positions of the atoms inside the unit cell are
described by the set of atomic positions (xi , yi , zi) measured from a lattice
point. Commonly, atomic positions are represented in terms of fractional
coordinates, relative to the unit cell lengths.
Simple cubic (P)
Body-centered cubic (I)
45
Face-centered cubic (F)
The atom positions within the unit cell can be calculated through application of
symmetry operations to the asymmetric unit. The asymmetric unit refers to the
smallest possible occupation of space within the unit cell. This does not, however
imply that the entirety of the asymmetric unit must lie within the boundaries of
the unit cell. Symmetric transformations of atom positions are calculated from
the space group of the crystal structure, and this is usually a black box operation
performed by computer programs.
The crystallographic directions are geometric lines linking nodes (atoms, ions or
molecules) of a crystal. Likewise, the crystallographic planes are geometric
planes linking nodes. Some directions and planes have a higher density of nodes.
These high density planes have an influence on the behavior of the crystal as
follows:[3]

Optical properties: Refractive index is directly related to density (or
periodic density fluctuations).

Adsorption and reactivity: Physical adsorption and chemical reactions
occur at or near surface atoms or molecules. These phenomena are thus
sensitive to the density of nodes.

Surface tension: The condensation of a material means that the atoms,
ions or molecules are more stable if they are surrounded by other similar
species. The surface tension of an interface thus varies according to the
density on the surface.
Grain boundaries
Grain boundaries are interfaces where crystals of different orientations meet.
A grain boundary is a single-phase interface, with crystals on each side of the
boundary being identical except in orientation. The term "crystallite boundary"
is sometimes, though rarely, used. Grain boundary areas contain those atoms
46
that have been perturbed from their original lattice sites, dislocations, and
impurities that have migrated to the lower energy grain boundary.
Treating a grain boundary geometrically as an interface of a single crystal cut
into two parts, one of which is rotated, we see that there are five variables
required to define a grain boundary. The first two numbers come from the unit
vector that specifies a rotation axis. The third number designates the angle of
rotation of the grain. The final two numbers specify the plane of the grain
boundary (or a unit vector that is normal to this plane).
Grain boundaries disrupt the motion of dislocations through a material, so
reducing crystallite size is a common way to improve strength, as described by
the Hall–Petch relationship. Since grain boundaries are defects in the crystal
structure they tend to decrease the electrical and thermal conductivity of the
material. The high interfacial energy and relatively weak bonding in most grain
boundaries often makes them preferred sites for the onset of corrosion and for
the precipitation of new phases from the solid. They are also important to many
of the mechanisms of creep.
Grain boundaries are in general only a few nanometers wide. In common
materials, crystallites are large enough that grain boundaries account for a small
fraction of the material. However, very small grain sizes are achievable. In
nanocrystalline solids, grain boundaries become a significant volume fraction of
the material, with profound effects on such properties as diffusion and plasticity.
In the limit of small crystallites, as the volume fraction of grain boundaries
approaches 100%, the material ceases to have any crystalline character, and thus
becomes an amorphous solid.
Defects and impurities
Real crystals feature defects or irregularities in the ideal arrangements described
above and it is these defects that critically determine many of the electrical and
mechanical properties of real materials. When one atom substitutes for one of
the principal atomic components within the crystal structure, alteration in the
electrical and thermal properties of the material may ensue. Impurities may also
manifest as spin impurities in certain materials. Research on magnetic impurities
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demonstrates that substantial alteration of certain properties such as specific
heat may be affected by small concentrations of an impurity, as for example
impurities in semiconducting ferromagnetic alloys may lead to different
properties as first predicted in the late 1960s. Dislocations in the crystal lattice
allow shear at lower stress than that needed for a perfect crystal structure.
The Bravais lattice is the basic building block from which all crystals can be
constructed. The concept originated as a topological problem of finding the
number of different ways to arrange points in space where each point would
have an identical ―atmosphere‖. That is each point would be surrounded by an
identical set of points as any other point, so that all points would be
indistinguishable from each other. Mathematician Auguste Bravais discovered
that there were 14 different collections of the groups of points, which are known
as Bravais lattices. These lattices fall into seven different "crystal systems‖, as
differentiated by the relationship between the angles between sides of the ―unit
cell‖ and the distance between points in the unit cell. The unit cell is the smallest
group of atoms, ions or molecules that, when repeated at regular intervals in
three dimensions, will produce the lattice of a crystal system. The ―lattice
parameter‖ is the length between two points on the corners of a unit cell. Each of
the various lattice parameters are designated by the letters a, b, and c. If two
sides are equal, such as in a tetragonal lattice, then the lengths of the two lattice
parameters are designated a and c, with b omitted. The angles are designated by
the Greek letters α, β, and γ , such that an angle with a specific Greek letter is
not subtended by the axis with its Roman equivalent. For example, α is the
included angle between the b and c axis.
Table shows the various crystal systems, while Figure shows the 14 Bravais
lattices. It is important to distinguish the characteristics of each of the individual
systems. An example of a material that takes on each of the Bravais lattices is
shown in Table.
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Geometrical characteristics of the seven crystal systems.
System
Axial lengths and angles
Unit cell geometry
cubic
a = b = c, α = β = γ = 90°
tetragonal
a = b ≠ c, α = β = γ = 90°
orthorhombic a ≠ b ≠ c, α = β = γ = 90°
rhombohedrala = b = c, α = β = γ ≠ 90°
hexagonal
a = b ≠ c, α = β = 90°, γ = 120°
monoclinic
a ≠ b ≠ c, α = γ = 90°, β ≠ 90°
triclinic
a ≠ b ≠ c, α ≠ β ≠ γ
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Examples of elements and compounds that adopt each of the crystal systems.
Crystal system
Example
triclinic
K2S2O8
monoclinic
As4S4, KNO2
rhombohedral
Hg, Sb
hexagonal
Zn, Co, NiAs
orthorhombic
Ga, Fe3C
tetragonal
In, TiO2
cubic
Au, Si, NaCl
The cubic lattice is the most symmetrical of the systems. All the angles are equal
to 90°, and all the sides are of the same length (a = b = c). Only the length of one
of the sides (a) is required to describe this system completely. In addition to
simple cubic, the cubic lattice also includes body-centered cubic and facecentered cubic (Figure). Body-centered cubic results from the presence of an
atom (or ion) in the center of a cube, in addition to the atoms (ions) positioned at
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the vertices of the cube. In a similar manner, a face-centered cubic requires, in
addition to the atoms (ions) positioned at the vertices of the cube, the presence of
atoms (ions) in the center of each of the cubes face.
The tetragonal lattice has all of its angles equal to 90°, and has two out of the
three sides of equal length (a = b). The system also includes body-centered
tetragonal (Figure).
In an orthorhombic lattice all of the angles are equal to 90°, while all of its sides
are of unequal length. The system needs only to be described by three lattice
parameters. This system also includes body-centered orthorhombic, basecentered orthorhombic, and face-centered orthorhombic (Figure). A basecentered lattice has, in addition to the atoms (ions) positioned at the vertices of
the orthorhombic lattice, atoms (ions) positioned on just two opposing faces.
The rhombohedral lattice is also known as trigonal, and has no angles equal to
90°, but all sides are of equal length (a = b = c), thus requiring only by one lattice
parameter, and all three angles are equal (α = β = γ ).
A hexagonal crystal structure has two angles equal to 90°, with the other angle (
γ ) equal to 120°. For this to happen, the two sides surrounding the 120° angle
must be equal (a = b), while the third side (c) is at 90° to the other sides and can
be of any length.
The monoclinic lattice has no sides of equal length, but two of the angles are
equal to 90°, with the other angle (usually defined as β) being something other
than 90°. It is a tilted parallelogram prism with rectangular bases. This system
also includes base-centered monoclinic (Figure).
In the triclinic lattice none of the sides of the unit cell are equal, and none of the
angles within the unit cell are equal to 90°. The triclinic lattice is chosen such
that all the internal angles are either acute or obtuse. This crystal system has the
lowest symmetry and must be described by 3 lattice parameters (a, b, and c) and
the 3 angles (α, β, and γ ).
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Crystalline Defect Types of Defects
Point defects (composition)
• Vacancies (missing atoms)
• Interstitials (extra atoms)
• Impurities (unwelcome segregation)
Extensive chemical changes
- Solid solutions
- Not a defect in intentional alloying or doping
o Line defects (1-dimensional) (Deformation)
• Dislocations in metals
Interfacial defects (2-dimensional) (Properties)
- surfaces - both interior (pore walls) and exterior (surface of material) interfaces
-(grain boundaries)
Bulk-Volume defects (3-dimensional)
- cracks, foreign inclusions, other phases (including pores).
Point Defects in Metals
Vacancy: An empty atomic site
Interstitial : An atom somewhere other than an atomic site
- Self-interstitial
- Impurity interstitial
Substitutional impurity: Some ―foreign‖ species on an atomic site
Line Defects: Dislocation in Metals
-Linear (one dimensional) defect around which some of the atoms are misaligned
Responsible for large mechanical deformation in crystalline solids
Types of Dislocations
Edge Dislocation:
A portion of an extra plane of atoms
- Screw Dislocation:
Helical atomic displacement around a line extending through the crystal
Mixed Dislocation: Some edge, some screw nature
Edge Dislocation, Screw Dislocation
Mixed Dislocation
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Mechanical Properties of Metals
Often materials are subject to external force when they are used. Mechanical
Engineers calculate those forces and material scientists how materials deform or
break as a function of force, time, temperature, and other conditions. Materials
scientists learn about these mechanical properties by testing materials.
Some of the important mechanical properties of a metals are Brittleness, Creep,
Ductility, Elasticity, Fatigue, Hardness, Malleability, Plasticity, Resilience,
Stiffness, Toughness, Yield strength. Above mechanical properties of metals are
explained below in brief.
Brittleness:
Tendency of a material to fracture or fail upon the application of a relatively
small amount of force, impact or shock.
Creep:
When a metal is subjected to a constant force at high temperature below its yield
point, for a prolonged period of time, it undergoes a permanent deformation.
Ductility:
Ductility is the property by which a metal can be drawn into thin wires. It is
determined by percentage elongation and percentage reduction in area of a
metal.
Elasticity:
Elasticity is the tendency of solid materials to return to their original shape after
being deformed.
Fatigue:
Fatigue is the of material weakening or breakdown of material subjected to
stress, especially a repeated series of stresses.
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Hardness:
Hardness is the ability of a material to resist permanent change of shape caused
by an external force.
Malleability:
Malleability is the property by which a metal can be rolled into thin sheets.
Plasticity:
Plasticity is the property by which a metal retains its deformation permanently,
when the external force applied on it is released.
Resilience:
Resilience is the ability of a metal to absorb energy and resist soft and impact
load.
Stiffness:
When an external force is applied on a metal, it develops an internal resistance.
The internal resistance developed per unit area is called stress. Stiffness is the
ability of a metal to resist deformation under stress.
Toughness:
When a huge external force is applied on a metal, the metal will experience
fracture. Toughness is the ability of a metal to resist fracture.
Yield strength:
The ability of a metal to bear gradual progressive force without permanent
deformation.
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How do metals respond to external loads?
Stress and Strain
Tension
Compression
Shear
Torsion
Elastic deformation
Plastic Deformation
Yield Strength
Tensile Strength
Ductility
Toughness
Hardness
To understand and describe how materials deform (elongate, compress, twist) or
break as a function of applied load, time, temperature, and other conditions we
need first to discuss standard test methods and standard language for
mechanical properties of materials. To understand and describe how materials
deform (elongate, compress, twist) or break as a function of applied load, time,
temperature, and other conditions we need first to discuss standard test methods
and standard language for mechanical properties of materials.
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Types of Loading
Tension
Compression
Torsion
Shear
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Concepts of Stress and Strain
(Tension and compression)
To compare specimens of different sizes, the load is calculated per unit area.
Stress:
F is load applied perpendicular to speciment crosssection; A is cross sectional
area (perpendicular to the force) before application of the load.
Strain:
Δl is change in length, lo is the original length
.
These definitions of stress and strain allow one to compare test results for
specimens of different cross sectional area A0 and of different length l0. Stress
and strain are positive for tensile loads, negative for compressive loads.
Concepts of Stress and Strain
(Shear and torsion)
Shear stress: τ
= F / Ao
F is load applied parallel to the upper and lower faces each of which has an area
A0
= the shear stress;
= the force applied;
= the cross-sectional area of material with area parallel to the applied force
vector.
Torsion is variation of pure shear. The shear stress in this case is a function of
applied torque T, shear strain is related to the angle of twist, φ.
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Stress-Strain Behavior
Elastic deformation
Reversible: when the stress is removed, the material returns to the dimensions it
had before the loading.
Usually strains are small (except for the case of some plastics, e.g. rubber).
Plastic deformation
Irreversible: when the stress is removed, the material does not return to its
original dimensions.
To compare specimens of different sizes, the load is calculated per unit area also
called normalized to the area . force divided by area is called stress
In tension and compression tests, the relevant area is that perpendicular to the
force
In shear or torsion tests, the area is perpendicular to the axis of rotation
S= F/ A tensile or compressive stress
T = F/ A shear stress
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There is a change in dimensions , or deformation elongation , DL as a result of
tensile or compressive stress . to enable comparison with specimens of different
length, the elongation is also normalized , this time to the length L this is called
strain.
e= DL / L
the change in dimensions is the reason we use A0 to indicate the initial area since
it changes during deformations. One could divide force by the actual area this is
called true stress.
Stress strain behavior
Elastic deformation , when the stress is removed the material returns to the
dimension it had before the load was applied . valid for small strains ( except the
case of rubbers )
Deformation is reversible non-permanent
Plastic deformation when the stress is removed the material does not return to its
pervious dimension but there is a permanent irreversible deformation.
In tensile tests if the deformation is elastic the stress – strain relationship is called
hook‘s law
S= Ee
That‘s E is the slope of the stress – strain curve. E is Young‘s modulus or
modulus of elasticity. in some cases the relationship is not linear so that E can be
defind alternatively as the local slope
E = ds/ de
Shear stresses produce strains according to
t= G g
where G is the shear modulus
elastic moduli measure the stiffness of the material. They are related to the
second derivative of the interatomic potential, or the first derivative of the force
vs internuclear distance . by examination these curves we can tell which material
has a higher modulus . due to thermal vibrations the elastic modulus decreases
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with temperature. E is large for ceramics (stronger ionic bond) and small for
polymers (weak covalent bond)
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