University of Technology
Department of Materials Engineering
3rd Grad
Nanotechnology (Lec.3+4)
By: Dr. Mayyadah S. Abed
2014-2015
Periodic table:
is a tabular arrangement of the chemical elements, organized on the basis of
their atomic number (number of protons in the nucleus), electron configurations,
and recurring chemical properties. Elements are presented in order of increasing
atomic number, which is typically listed with the chemical symbol in each box.
The standard form of the table consists of a grid of elements laid out in 18 columns
and 7 rows, with a double row of elements below that. The table can also be
deconstructed into four rectangular blocks: the s-block to the left, the p-block to
the right, the d-block in the middle, and the f-block below that.
The rows of the table are called periods; the columns are called groups, with
some of these having names such as halogens or noble gases. Since, by definition,
a periodic table incorporates recurring trends, the table can be used to derive
relationships between the properties of the elements and predict the properties of
new, yet to be discovered or synthesized, elements. As a result, a periodic table
provides a useful framework for analyzing chemical behavior, and so the tables, in
various forms, are widely used in chemistry and other sciences.
Atomic Number (Z)
The atomic number is equal to the number of protons in an atom's nucleus. The
atomic number determines which element an atom is. For example, any atom that
contains exactly 47 protons in its nucleus is an atom of silver.
The number of neutrons in atom can change, when this happens it is said to be a
different isotope of the element.
Mass Number (A)
The mass number, symbol A, also called atomic mass number (not to be
confused with atomic number (Z)) or nucleon number, is the number of nucleons
(protons and neutrons) in an atomic nucleus. The mass number is unique for each
isotope of an element and is written either after the element name or as a
superscript to the left of an element's symbol. Carbon-12 (12C) has 6 protons (as
do ALL isotopes of carbon) and 6 neutrons, 6+6 = 12.
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University of Technology
Department of Materials Engineering
3rd Grad
Nanotechnology (Lec.3+4)
By: Dr. Mayyadah S. Abed
2014-2015
Lead-207 (207Pb) has 82 protons (as do ALL isotopes of lead) and 125 neutrons,
82+125 = 207.
Neutrons (N)
Neutrons are uncharged particles found within atomic nuclei. Neutrons were
discovered by James Chadwick in 1932. The number of neutrons determines the
isotope of an element. (For example, the carbon-12 isotope has 6 protons and 6
neutrons, while the carbon-14 isotope has 6 protons and 8 neutrons.) Isotopes are
atoms of the same element that have the same atomic number but different masses
due to a different number of neutrons. A neutron is classified as a baryon, and
consists of two down quarks and one up quark. The proper symbol to use when
representing a neutron;
Atomic weight
An atomic weight (relative atomic mass) of an element from a specified
source is the ratio of the average mass per atom of the element to 1/12 of the mass
of 12C in its nuclear and electronic ground state. A sample of any element consists
of one or more isotopes of that element. Each isotope is a different weight.
The relative amounts of each isotope for any element represents the isotope
distribution for that element. The atomic weight is the average of the isotope
weights weighted for the isotope distribution and expressed on the 12C scale as
mentioned above. The standard atomic weights apply to the elements as they exist
naturally on Earth, and the uncertainties take into account the isotopic variation
found in most laboratory samples.
2
University of Technology
Department of Materials Engineering
3rd Grad
Nanotechnology (Lec.3+4)
By: Dr. Mayyadah S. Abed
2014-2015
3
3rd Grad
Nanotechnology (Lec.3+4)
By: Dr. Mayyadah S. Abed
University of Technology
Department of Materials Engineering
2014-2015
Orbital hybridization
In chemistry, hybridisation (or hybridization)
is
the
concept
of
mixing atomic orbitals into new hybrid orbitals (with different energies, shapes,
etc., than the component atomic orbitals) suitable for the pairing of electrons to
form chemical bonds in valence bond theory. Hybrid orbitals are very useful in the
explanation of molecular geometry and atomic bonding properties.
Types of hybridisation
1. sp3 hybrids
Four sp3 orbitals.
Hybridisation describes the bonding atoms from an atom's point of view. That is,
for a tetrahedrally coordinated carbon (e.g., methane CH 4 ), the carbon should have
4 orbitals with the correct symmetry to bond to the 4 hydrogen atoms.
Carbon's ground state configuration is 1s2 2s2 2p x 1 2p y 1 or more easily read:
↑↓ ↑↓ ↑
↑
C
1s 2s 2p x 2p y 2p z
The carbon atom can utilize its two singly occupied p-type orbitals (the
designations p x p y or p z are meaningless at this point, as they do not fill in any
particular order), to form two covalent bonds with two hydrogen atoms, yielding
the "free radical" methylene CH 2 , the simplest of the carbenes. The carbon atom
can also bond to four hydrogen atoms by an excitation of an electron from the
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3rd Grad
Nanotechnology (Lec.3+4)
By: Dr. Mayyadah S. Abed
University of Technology
Department of Materials Engineering
2014-2015
doubly occupied 2s orbital to the empty 2p orbital, so that there are four singly
occupied orbitals.
↑↓ ↑ ↑
↑
↑
C*
1s 2s 2p x 2p y 2p z
As the energy released by formation of two additional bonds more than
compensates for the excitation energy required, the formation of four C-H bonds is
energetically favoured.
Quantum mechanically, the lowest energy is obtained if the four bonds are
equivalent which requires that they be formed from equivalent orbitals on the
carbon. A set of four equivalent orbitals can be obtained which are linear
combinations of the valence-shell (core orbitals are almost never involved in
bonding) s and p wave functions which are the four sp3 hybrids.
↑↓ ↑ ↑ ↑ ↑
C*
1s sp3 sp3 sp3 sp3
In CH 4 , four sp3 hybrid orbitals are overlapped by hydrogen 1s orbitals, yielding
four σ (sigma) bonds (that is, four single covalent bonds) of equal length and
strength.
5
3rd Grad
Nanotechnology (Lec.3+4)
By: Dr. Mayyadah S. Abed
University of Technology
Department of Materials Engineering
2014-2015
2. sp2 hybrids
Three sp2 orbitals.
Ethene structure
Other carbon based compounds and other molecules may be explained in a similar
way as methane. For example, ethene (C 2 H 4 ) has a double bond between the
carbons.
For this molecule, carbon will sp2 hybridise, because one π (pi) bond is required
for the double bond between the carbons, and only three σ bonds are formed per
carbon atom. In sp2 hybridisation the 2s orbital is mixed with only two of the three
available 2p orbitals:
↑↓ ↑ ↑ ↑ ↑
C*
1s sp2 sp2 sp2 2p
forming a total of three sp2 orbitals with one p orbital remaining. In ethylene
(ethene) the two carbon atoms form a σ bond by overlapping two sp2orbitals and
each carbon atom forms two covalent bonds with hydrogen by s–sp2 overlap all
with 120° angles. The π bond between the carbon atoms perpendicular to the
molecular plane is formed by 2p–2p overlap. The hydrogen–carbon bonds are all
of equal strength and length, which agrees with experimental data.
sp hybrids
6
3rd Grad
Nanotechnology (Lec.3+4)
By: Dr. Mayyadah S. Abed
University of Technology
Department of Materials Engineering
2014-2015
Two sp orbitals
The chemical bonding in compounds such as alkynes with triple bonds is explained
by sp hybridisation.
↑↓ ↑ ↑ ↑ ↑
C*
1s sp sp 2p 2p
In this model, the 2s orbital mixes with only one of the three p orbitals resulting in
two sp orbitals and two remaining unchanged p orbitals. The chemical bonding
in acetylene (ethyne) (C 2 H 2 ) consists of sp–sp overlap between the two carbon
atoms forming a σ bond and two additional π bonds formed by p–p overlap. Each
carbon also bonds to hydrogen in a σ s–sp overlap at 180° angles.
Conclusion:
Diamond: SP3
C-C
Graphite, graphene, CNT, fullerene: SP2
Alkyne (acetylene C2H2): SP
C=C+ C-C
CΞC
7
University of Technology
Department of Materials Engineering
3rd Grad
Nanotechnology (Lec.3+4)
By: Dr. Mayyadah S. Abed
2014-2015
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