The Valence Bond Theory (VBT) was developed to overcome limitations of the Lewis concept. According to VBT: - Bonds form due to the overlap of half-filled valence shell orbitals between atoms. Sigma bonds form from axial orbital overlap along the internuclear axis, while pi bonds form from lateral orbital overlap perpendicular to this axis. - Hybridization of atomic orbitals can explain the geometry of molecules like BF3, CH4, and PCl5, whose bond angles differ from the angle between pure atomic orbitals. Hybridization involves combining orbitals to form new hybrid orbitals that better describe molecular geometry.

2. 2

3. To overcome Limitations of Lewis Concept, Valence Bond Theory (VBT) with concept of
Hybridization was put forth.
Assumptions of VBT:
 Valence shell orbitals of atoms in molecule overlap with each other to form bond. Half
filled orbitals overlap with each other to form bond.
 Number bonds formed are equal to number of half filled orbital in an atom.
 Linear /Axial overlap of orbitals form sigma bond.
 Lateral / side wise overlap of orbitals form pi bond.
Sigma bond is very stronger than pi bond.
 Orbitals overlap with each with similar sign of wave
function.
 Force of attractions is due to orbital overlap.
 Geometry of molecule can be explained on the basis
Of hybridization of atomic orbitals. 3

4. -
Internuclear axis
Axial overlap of orbitals
+
+ + + +
s Orbital overlap linearly to form sigma bond
Internuclear axis
z
+ - z
y
+ Internuclear axis
pz orbitals of two atoms can overlap linearly to form sigma (σ) bond
(z axis is considered as internuclear axis – a convention by IUPAC)
Side-wise overlap of orbitals
y y y y
P orbitals of two atoms can overlap side-wise to form pi(π) bond.
4

5.  Electronic configuration of ‘H’atom is 1s1
 H atom consists of half filled 1s orbital which take part in bonding.
 Half filled 1s orbital of one H atom overlap with Half filled 1s orbital of another H atom to form H-
H sigma bond.
Example of VBT: Formation of H2 Molecule
+
+ + + +
1s Orbital of H atom overlap linearly to form sigma bond
Internuclear axis
H2 Molecule consists of H– H sigma bond
It has linear structure.
5

6.  Electronic configuration of 'O’atom is 1s2, 2s2, 2px
2 2py
1 2pz 1
 py orbital and pz orbital are half filled.
 If the pz orbital of one 'O' atom overlaps with the pz orbital of other 'O' atom along
internuclear axis to form σ bond.
Example of VBT: Formation of O2 Molecule
 pz – pz orbital overlap is along internuclear axis.
 This type of overlap results into the formation
σ bond between two oxygen atoms.
Pz – pz axial overlap
+
+
pz
+
pz
6

7. One05i/0s8/σ202b0ondanother is π bond.
 In in oxygen molecule double bond is formed (O=O).
+ Internuclear axis
 If the py orbital of one 'O' atom overlaps with the py orbital of other 'O' atom perpendicular to
internuclear axis, py – py overlap and π bond is formed.
y y y y
Py orbitals of two O atoms overlap side-wise manner to form Pi (π) bond
 Overall orbital overlapping is -
+
σ - bond
py
pz
-
py
pz
-
7

8.  Geometry of some molecules such as O2, N2, H2, HF, etc. can be explained on the basis of
VBT. But geometry of many molecules cannot be explained on the basis of orbital overlap
of pure atomic orbitals. Examples are -BF3 have trigonal planar structure. CH4 have
Tetrahedral Structure. PCl5 have trigonal bipyramidal structure, etc.
Since bond angel is different than angle between pure orbitals. Examples: In BF3 bond
angle is 120˚ while angle between orbitals is 90˚. In CH4 bond angle is 109˚ 28˝ while
angle between orbitals is 90˚.
BF3
CH4
To explain structure of these molecules the concept of
PCl5
05/08/2020
hybridization is introduced in VBT
. 8

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