Lecture 7
Atomic Structure
- Introductions
Diamond and graphite both consist of pure carbon, but their materials
properties vary different. These differences arise from differences in the
arrangements of the atoms in the solids and differences in the bonding
between atoms.
Covalent bonding in diamond leads to high strength and stiffness,
excellent thermal conductivity, and poor electrical conductivity.
The atoms in graphite are arranged in sheets. Between the sheets, the
bonds are less strong. Thus graphite can easily be slide off in papers as
occurs when writing with a pencil.
Graphite’s thermal conductivity is much lower than that of diamond, and
its electrical conductivity is much higher.
- Fundamental Concepts
Each atom consists of a very small nucleus composed of protons and
neutrons, and electrons moving around atom.
Both electrons and protons are electrically charged, the charge magnitude
being 1.602 * 10-19,
Electrons are negative in sign and protons are positive. Neutrons are
electrically neutral.
Masses for these particles are very small, protons and neutrons have
approximately the same mass, 1.67 * 10
electron, 9.11 *10-31 kg.
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, which is larger than that of an
Lecture 7
Atomic Structure
Atomic Number (Z): The atomic number also equals the number of
electrons. This atomic number ranges in integral units from 1 for hydrogen
to 92 for uranium.
The atomic mass (A) :atomic mass of a specific atom may be expressed
as the sum of the masses of protons and neutrons within the nucleus.
Number of protons is the same for all atoms of a given element, the
number of neutrons (N) may be variable Thus atoms of some elements have
two or more different atomic masses, which are called isotopes.
- Electronic Structure of the Atom
Electrons occupy an energy levels within the atom. Each electron
possesses a particular energy
No more than two electrons in each atom having the same energy.
Figure representation of the Bohr atom.
 Quantum Numbers
The energy level to which each electron belongs is identified by four
quantum numbers.
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Lecture 7
Atomic Structure
Principal quantum number (n), the azimuthal or secondary quantum
number (l), the magnetic quantum number (ml), and the spin quantum
number (ms).
The principal quantum number (n) reflects the grouping of electrons
into energy levels known as shells.
It is describe by the letters K, L, M, N, O, and so on. Which correspond,
respectively, to n = 1, 2, 3, 4, 5 . . .,
This quantum number is related to the distance of an electron from the
nucleus, or its position.
Secondary quantum numbers (l) indicates the subshell (number of
orbitals), which is denoted by a lowercase letter an s, p, d, or f; it is related
to the shape of the electron subshell. In addition, the number of these
subshells is limited by the magnitude of n, (l = n – 1) so for s=0, p=1. d=2,
f=3 .
Magnetic quantum number (ml) gives the number of energy levels, or
orbitals, for each secondary quantum number (ml = 2l + 1).
Values for ml are given by whole numbers between -l and +l. For example,
if l = 2, there are 2(2) + 1 = 5 magnetic quantum numbers with values -2,
-1, 0, +1, and +2.
Spin quantum number (ms) is indicated by values of +1/2 and -1/2,
which reflect values of “spin” of an electron.
Example: in the K shell (that is, n = 1), there is just a single (s) orbital (as
the only allowable value of l is 0 and ml is 0). As a result, a K shell may
contain no more than two electrons.
As another example, for M shell., n = 3, so l takes values of 0, 1, and 2,
(there are s, p, and d orbitals present). The values of ml reflect that there is
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Lecture 7
Atomic Structure
a single s orbital (ml = 0, a single value), three p orbitals (ml = -1, 0, +1, or
three values), and five d orbitals (ml =-2, -1, 0, 1, 2, or five discrete values).
Table below illustrate more example for quantum number of electrons
 Electron Configurations
As we know all electrons uniformly distributed among orbitals of atom,
that can be known by The Aufbau Principle
Aufbau Principle is a graphical shape that predicts configuration of
electrons from the expected ordering of the energy levels. The Aufbau
principle is shown in Figure below
1s
2s
3s
4s
5s
6s
7s
2p
3p
4p
5p
6p
7p
3d
4d
5d
6d
7d
4f
5f 5g
6f 6g 6h
7f 7g 7h 7i
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Lecture 7
Atomic Structure
For example, according to the Aufbau Principle, the electronic structure
of iron, atomic number 26, is
1s22s22p63s23p64s23d6
Conventionally, the principal quantum numbers are arranged from lowest
to highest when writing the electronic structure. Thus, the electronic
structure of iron is written
1s22s22p63s23p63d6 4s2
Note that not all elements follow the Aufbau principle. A few, such as
copper, are exceptions. According to the Aufbau Principle, copper should
have the electronic structure
1s22s22p63s23p63d94s2,
but copper actually has the electronic structure
1s22s22p63s23p63d10 4s1
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