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Valence Bond Theory of Homonuclear Diatomic Molecules
1. CHAPTER 2: VALENCE BOND THEORY
CHEM210/Chapter 2/2014/01
HOMONUCLEAR DIATOMIC MOLECULES: VALENCE
BOND (VB) THEORY
The word homonuclear is used in two ways:
• A homonuclear covalent bond is formed between
atoms of the same element, e.g. the H – H bond in
H2, the O = O bond in O2 and the O – O bond in
H2O2.
• A homonuclear molecule contains
one type of element, e.g. H2, N2 and
F2 and larger molecules such as O3,
P4, S8 and C60.
2. CHEM210/Chapter 2/2014/02
Before discussing covalent bonding, we consider the following:
• For an atom X, the value of the single bond covalent radius, rcov, is half the
internuclear separation in a homonuclear X – X bond.
• The van der Waals radius, rv, of an atom X is half of the distance of closest
approach of two non-bonded atoms of X
In covalent bonding, as two nuclei approach each other their atomic orbitals
overlap.
• As the amount of overlap increases, the energy of the interaction decreases.
• At some distance the minimum energy is reached.
• The minimum energy corresponds to the bonding distance (or bond length).
• As the two atoms get closer, their nuclei begin to repel and the energy
increases.
At the bonding distance, the attractive forces between nuclei and electrons just
balance the repulsive forces (nucleus-nucleus, electron-electron).
4. CHEM210/Chapter 2/2014/04
A covalent bond forms when the orbitals of two atoms overlap and the overlap
region, which is between the nuclei, is occupied by a pair of electrons.
A set of overlapping orbitals has a maximum of two electrons that must have
opposite spins.
The greater the orbital overlap, the stronger (more stable) the bond.
The valence atomic orbitals in a molecule are different from those in isolated
atoms.
There is a hybridization of atomic orbitals to form molecular orbitals.
6. CHEM210/Chapter 2/2014/06
HYBRIDIZATION
The number of hybrid orbitals obtained equals the number of atomic orbitals
mixed.
The type of hybrid orbitals obtained varies with the types of atomic orbitals
mixed.
sp hybrid orbital
The sp hybrid orbitals in gaseous BeCl2.
atomic
orbitals
hybrid
orbitals
8. CHEM210/Chapter 2/2014/08
sp2 hybrid orbitals
All molecules with trigonal planar electron pair geometries have sp2 orbitals
on the central atom.
When we mix n atomic orbitals we must get n hybrid orbitals.
sp2 hybrid orbitals are formed with one s and two p orbitals, therefore, there
is one unhybridized p orbital remaining.
The large lobes of sp2 hybrids lie in a trigonal plane.
The sp2 hybrid orbitals in BF3.
9. CHEM210/Chapter 2/2014/09
sp3 hybrid orbitals
sp3 hybrid orbitals are formed from one s and three p orbitals, therefore, there
are four large lobes.
Each lobe points towards the vertex of a tetrahedron.
The angle between the large lobes is 109.5
The sp3 hybrid orbitals in CH4.
12. CHEM210/Chapter 2/2014/12
Geometrical arrangements characteristic of hybrid orbital sets
Atomic orbital
set
Hybrid orbital
set
Geometry Examples
s, p Two sp BeF2, HgCl2
s, p, p Three sp2 BF3, SO3
s, p, p, p Four sp3 CH4, NH3,
H2O, NH4
+
14. CHEM210/Chapter 2/2014/14
MULTIPLE BONDS
Have and -bonds.
In -bonds, the electron density lies on the axis between the nuclei. All single
bonds are -bonds.
-Bonds: electron density lies above and below the plane of the nuclei.
A double bond consists of one -bond and one -
bond.
A triple bond has one -bond and two -
bonds.
Often, the p-orbitals involved in -bonding
come from unhybridized orbitals.
15. CHEM210/Chapter 2/2014/15
Ethane, C2H6
both Cs are sp3
hybridized
s-sp3 overlaps to σ
bonds
sp3-sp3 overlap to
form a σ bond
relatively even distribution of
electron density over all σ bonds
16. CHEM210/Chapter 2/2014/16
Ethylene, C2H4
One - and one -bond with both C atoms being sp2 hybridized.
Both C atoms with trigonal planar electron pair and molecular geometries.