This document discusses the molecular orbital theory presented by Dr. Farhat A. Ansari, Assistant Professor at JETGI. It introduces key concepts of molecular orbital theory including that atomic orbitals combine to form molecular orbitals belonging to the whole molecule. Molecular orbitals can be constructed through a linear combination of atomic orbitals. The document provides rules for linear combination and uses of molecular orbitals, and examples of applying molecular orbital theory to diatomic molecules including H2, He2, Li2, and B2.
2. Molecular Orbital Theory
Band Theory of Solids
Liquid Crystal & Its Application
Point Defects in Solids
Structure of Graphite & Fullerene
UNIT-I
3. Molecular Orbital Theory
1. MO theory suggests that atomic orbitals of different
atoms combine to create MOLECULAR ORBITALS
2. Electrons in these MOLECULAR ORBITALS belong to the
molecule as whole
3. This contrasts to VB theory which suggests that electrons
are shared by simple overlap atomic orbital's or
hybridized atomic orbital's .
4. Molecular orbital can be constructed from linear
combination of atomic orbital's
MO = LCAO
INTRODUCTION
4. Rules for linear combination
1. Atomic orbital's must be roughly of the same energy.
2. The orbital must overlap one another as much as
possible- atoms must be close enough for effective
overlap.
3. In order to produce bonding and antibonding MOs,
either the symmetry of two atomic orbital must remain
unchanged when rotated about the internuclear line or
both atomic orbital's must change symmetry in identical
manner.
Linear combination of atomic orbitals
5. Rules for the use of MOs
* When two AOs mix, two MOs will be produced
* Each orbital can have a total of two electrons (Pauli
principle)
* Lowest energy orbitals are filled first (Aufbau principle)
* Unpaired electrons have parallel spin (Hund’s rule)
Bond order = ½ (bonding electrons – antibonding electrons)
6. A B
A B
AB = N(cA A + cBB)
Linear Combination of Atomic Orbitals (LCAO)
2AB = (cA2 A2 + 2cAcB A B + cB2 B 2)
Overlap integral
The wave function for the molecular orbitals can be approximated by
taking linear combinations of atomic orbitals.
Probability density
c – extent to which each AO
contributes to the MO
7. When 2 atomic orbitals combine there are 2
resultant orbitals.
low energy bonding orbital
high energy antibonding orbital
1sb 1sa
s1s
s*
E
1s
Molecular
orbitals
Eg. s orbitals
8. First period diatomic molecules
s1s2H
Energy
HH2
1s 1s
sg
su*
Bond order = ½ (bonding electrons
– antibonding electrons)
Bond order: 1
9. s1s2, s*1s2He
Energy
HeHe2
1s 1s
sg
su*
Molecular Orbital theory is powerful because it allows us to predict
whether molecules should exist or not and it gives us a clear picture
of the of the electronic structure of any hypothetical molecule that
we can imagine.
Diatomic molecules: The bonding in He2
Bond order: 0
10. Second period diatomic molecules
s1s2, s*1s2, s2s2
Bond order: 1
Li
Energy
LiLi2
1s 1s
1sg
1su*
2s 2s
2sg
2su*
11. s1s2, s*1s2, s2s2,
s*2s2
Bond order: 0
Be
Energy
BeBe2
1s 1s
1sg
1su*
2s 2s
2sg
2su*
Diatomic molecules: Homonuclear Molecules
of the Second Period
