VALENCE BOND THEORY

Valence Bond Theory (VB – T)

•Introduced by Heilter and London (1927), after which this theory also known as London Forces.

•Based on Atomic Orbital Concept and E.C. of atoms.

•Was further developed by Nobel laureate Prof. Linus Pauling.

•The model tells us that Covalent Bond is formed when two orbital of the combining atoms overlap to produce a new combined orbital containing 2 e– of opposite spins.

•The resulting overlapping results in the decrease in the Energy of the atoms forming the bond.

•The shared e pair is most likely to be found in the space between the 2 nuclei of the combining atoms.

Example of H2 molecule using VB – Ts – s Overlapping

More Explanation on Orbital Overlapping Concept (taking H2 as example)

• As two nuclei of the 2 H2 atoms come closer for the formation of a

bond, new attractive and repulsive forces begin to operate.

• The nucleus of one atom is attracted (Attractive Forces) towards its

own electron and the electron of the other atom and vice versa.

• Repulsive Forces arise between the e– of the 2 atoms and the nuclei

of the 2 atoms.

• However, Attractive Forces are more compared to Repulsive Forces,

hence, the atoms come together forming a bond.

• The H – H bond energy of 435.8 kJ/mol makes it very stable

compared to single H atom.

According to Orbital Overlapping Concept, when atoms combine and overlap to

form covalent bond, they possess e– having opposite spins.

Formation of HF Bond by combination of 1 F and 1 H atom:

Z of F = 9 , E.C. of F = 1s2 2s2 2px2 2py

2 2pz1

Z of H = 1, E.C. of H = 1s1

Types of Orbital OverlapDepending on the type of overlapping, the covalent bonds are of the following

types:

1. Sigma Bond (σ) 2. Pi Bond (π)

Sigma Bond (σ): (Formed by Axial Overlapping)Is formed by end to end overlapping (head – on overlap) of bonding orbital along

the interneuclear axis. These are of 3 types:

1. s – s overlapping: in this case there is overlapping of 2 half – filled s – orbitals

along the internuclear axis

2. s – p overlapping: in this case there is overlapping between half

filled s – orbital of one atom and half – filled p – orbital of the other

atom.

3. p – p overlapping: in this case there is overlapping of half – filled

p – orbitals of the two approaching atoms.

Pi Bond (π): Sideways Overlapping

Is formed by the overlapping of atomic orbitals when they overlap in

a way that their axes remain parallel to each other and perpendicular

to the internuclear axis.

ORBITAL HYBRIDIZATION

Hybridization

• It’s the process of intermixing of the orbitals (Hybrid) of the combining atoms of

slightly different energies. They form different shapes of orbitals after combining.

• Orbitals of Valence Shells of almost similar energies can participate.

• Number of hybrid orbitals produced depends on the number of atomic orbitals

participating.

• Geometry of the molecule formed can be determined by its type of

Hybridization.

• The hybrid orbitals are more effective in forming stable bonds than pure atomic

orbitals.

• Exceptionally, in some cases filled orbitals of valence shell also sometimes take

part in hybridization.

Types of Hybridization

sp Hybridization: When 1 s and 1p orbital hybridize to form 2 equivalent

orbitals, it is known as sp Hybridization.

Each of the 2 hybrid orbitals will possess 50% s character and 50% p character.

Ex: Acetylene (C2H2), BeCl2, BeF2, BeH2 etc.

sp2 Hybridization: When 1 s and 2 p orbitals hybridize to form 3

equivalent orbitals, it is known as sp2 Hybridization.

All the 3 hybrid orbitals remain in the same plane making an angle of 1200

Eg: Compounds like BF3, BH3, BCl3 etc. and those compounds containing Carbon

Carbon double bonds.

sp3 Hybridization: When 1 s and 3 p orbitals hybridize in the valence shell

to form 4 equivalent hybrid orbitals, it is known as sp3 Hybridization.

There is 25% s character and 75% p character in each sp3 hybrid orbital.

The four hybrid orbitals remain directed towards the four corners of a regular

tetrahedron.

It generally makes an angle of 109.50

Eg: Compounds like NH3 and H2O exhibit such hybridization.

Hybridization involving s, p, and d orbitals

There are 2 such types:

sp3d Hybridization: When 1 s, 3 p and 1 d orbitals of the valence shells are

involved in hybridization, it gives rise to 5 equivalent orbitals.

Out of the 5, 3 are directed towards the corners of an equilateral triangle, whereas,

2 of them are perpendicular to the plane of the triangle. The former 3 create an

angle of 1200.

Eg: PCl5 molecule behave much the same way as sp3d Hybridization.

sp3d2 Hybridization: When 1 s, 3 p and 2 d orbitals of the valence shells

hybridize, it gives 6 new equivalent hybrid orbitals, which are projected towards the

6 corners of a regular octahedron.

For example, molecules like SF6 behave in much the same way.

sp3d3 Hybridization: When 1 s, 3 p and 3 d orbitals of the valence

shell hybridize, it forms 7 new equivalent hybrid orbitals, which are projected

towards the seven corners of a regular pentagonal bipyramid.

Eg: Molecules like IF7 exhibit such geometry.