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Structure and Properties of Solids
• Chemical bond: attractive force holding two or more atoms together.
• Covalent bond results from sharing electrons between the atoms. Usually found between nonmetals.
• Ionic bond results from the transfer of electrons from a metal to a nonmetal.
• Metallic bond: attractive force holding pure metals together.
Arrangement of atoms in solids
1.
2.
3.
4.
Metallic Bonding
Chemists use the electron-sea model to describe metallic
bonding. The model proposes that the valence electrons of
metal atoms move freely among the ions, forming a “sea” of
delocalized electrons that hold the metal ions rigidly in place.
Microscopic analysis shows that the
structure of metals consists of aggregates
of crystals.
Metallic Crystals
Covalent Crystals
Ionic Crystals
Molecular Crystals
1. Metallic Crystals
• Individual metal atoms of metallic crystals sit on lattice sites. This leaves the outer electrons of these atoms free to float around the lattice. Metallic crystals tend to be very dense and have high melting points. 1
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Properties of Metals
Melting and Boiling Points
• the stronger the bonding forces, the higher the melting and
boiling points of pure metals
Properties of Metals
Electrical and Thermal Conductivity
• Metals are good conductors because their electrons are
free to move from one atom to the next.
Malleability and Ductility
• Based on the electron-sea model, metals can be shaped
because, when struck, the metal ions can slide by one
another while the electrons still surround them.
Periodic table trends include:
1. For Group 1, melting points decrease as the atomic number increases.
2. For Groups 1 to 6, across a period, melting points increase as atomic number increases.
Hardness
• The variation between metals is due to differences in
crystal size (smaller ones make harder metals).
UNIT 2 Chapter 4: Chemical Bonding and Properties of Matter Section 4.1
Alloys
Alloys are solid mixtures of two or more metals.
• the addition of the second metal, even in a very small amount,
can significantly affect the properties of a substance
• in some cases, non-metal atoms, such as carbon, are added
If atoms of the second
metal are similar in size to
the first metal, they take the
place of those atoms.
2. Covalent Crystals
• Network solids are macromolecules, giant structures of covalently bonded atoms in one, two or three dimensional arrays.
If atoms of the second metal are
much smaller than atoms of the first
metal, they will fit in spaces
between the larger atoms.
Covalent Crystals
• A covalent crystals has true covalent bonds between all of the atoms in the crystal. You can think of a covalent crystal as one big molecule. Many covalent crystals have extremely high melting points. Examples of covalent crystals include diamond and zinc sulfide crystals. 2
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UNIT 2 Chapter 4: Chemical Bonding and Properties of Matter Section 4.1
Allotropes
Allotropes are compounds that consist of the same element
but have different arrangements of the atoms, and therefore
different physical properties.
An example is allotropes of carbon, which differ in the
pattern of covalent bonds between carbon atoms.
Network Solids of Silicon
• Silicon makes up 25.7 % of the Earth’s crust.
1. Quartz • A large 3‐dimensional network with silicon and oxygen.
• Each silicon is tetrahedrally bonded to four oxygen atoms.
• Foreign metal ions in quartz produce semiprecious stones such as emerald, amethyst and garnet.
O
O
O
Si
O
Allotropes of carbon: A graphite, B diamond,
C buckyballs, D nanotubes
O
Si
O
O
O
Si
O
O
Graphite
2‐Dimensional Network Solids
• Networks which form 2‐D arrays or sheets.
• Within a sheet the atoms are held together with covalent bonds.
• Weak van der Waals forces hold the layers together.
• These solids will also have high melting and boiling points however they will be soft and the layers will slide over each other allowing them to be used as a lubricant.
• An example of this type of solid is graphite.
• Graphite is soft, slippery and is used as a lubricant because the layers can slide past each other.
• The delocalized electrons give stability to graphite.
• The delocalized electrons are able to move freely therefore allowing graphite to conduct electricity.
• When you write with a pencil you are breaking off layers of the graphite.
C
C
120°
C
C
C
C
C
C
C
C
C
C
• Graphite is a 2‐dimensional array of carbon atoms arranged in layers of hexagons. • The electrons making up the “double bonds” are actually delocalized throughout the structure (i.e. not true double bonds)
• The layers of hexagons are held in place by van der Waals forces (intermolecular forces).
Mica
• 2‐dimensional sheets of silicate
• weak attractions between the layers make mica flake easily
• Mica is added to paint and makeup to provide sheen, to drywall compound to make it sandable, is used as window 'glass' in stoves and furnaces, used in electronics to make capacitors and insulators, in the plastic industry as an extender, filler, and reinforcing agent, and in the rubber industry as a filler.
O Si
2-
O
Si
O
Si
O Si
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1‐Dimensional Network Solids
• These are solids that form networks in a one dimensional array or fibre.
• They consist of long chains held together by covalent bonds.
• The forces between adjacent chains are very weak therefore the solids will form threads.
• They have very high melting and boiling points due to the strong covalent bonds.
• They are solids at room temperature and are not soluble in water.
• An example of this type of solid is asbestos.
Asbestos
• Asbestos forms fibrous, stringy one-dimensional
chains.
• Can be used as a fire retardant but it is now
known to cause lung cancer
+2
O- Ca O-
Si O
Si
O
Si
O Si
O- Mg+2 O-
UNIT 2 Chapter 4: Chemical Bonding and Properties of Matter Section 4.1
3. Ionic Crystals
• The atoms of ionic crystals are held together by electrostatic forces (ionic bonds). Ionic crystals are hard and have relatively high melting points. Table salt (NaCl) is an example of this type of crystal.
Ionic Crystals
Ionic compounds exist as crystal lattice structures with
particular patterns of alternating positive and negative ions.
The unit cell is the smallest group of ions that is repeated.
NaCl forms a cubic
crystal lattice structure.
Different types of crystal structures can form.
• the relative sizes and charges of the ions affect the type of
crystal structure that an ionic compound will form.
UNIT 2 Chapter 4: Chemical Bonding and Properties of Matter Section 4.1
Properties of Ionic Compounds
Crystal Shapes
Melting and Boiling Points
• high due to very strong attractions between ions
Solubility
• ionic compounds are soluble in water when the attractive
forces between the ions and water molecules are stronger
than the attractive forces among the ions themselves
When sodium chloride (NaCl)
dissolves in water, attractive
forces between water
molecules and NaCl ions act
to break apart the ionic bonds.
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UNIT 2 Chapter 4: Chemical Bonding and Properties of Matter Section 4.1
The Properties of Ionic Compounds
Properties of Ionic Compounds
Mechanical Properties
• hard and brittle, so will break apart when struck
Ionic crystal will break on
smooth planes, where like
charges become aligned.
Conductivity
• solids do not conduct because ions cannot move
• compounds conduct when dissolved in water and ions can
move
• Molten ionic compounds can conduct electricity because
their ions are free to move
4. Molecular Crystals
• These crystals contain recognizable molecules within their structures. A molecular crystal is held together by intermolecular forces, like van der Waals forces or hydrogen bonding. Molecular crystals tend to be soft with relatively low melting points. Rock candy, the crystalline form of table sugar or sucrose, is an example of a molecular crystal.
Properties of Covalent Compounds
Polarity is very important concept which helps us understand the physical properties of many chemicals and processes involving them.
Covalent compounds can form crystals based on intermolecular forces between the molecules.
Example:
Boiling point‐ two geometric isomers of C2H2Cl2 exist. One is polar and one is nonpolar. The polar isomer has the higher boiling point because of increased intermolecular interactions.
Properties of Covalent Compounds
Properties of solutions: the interaction of different covalent molecules is dominated by differences or similarities in polarity.
Example: the action of soap molecules. How does soap work?
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