This ppt explains the formation of molecules on the basis of Valence Vond theory.

1. Bonding in Molecules
Part-II: Valence Bond Theory
(Remedial Teaching Lecture-2)
Prepared by
D. R. Shinde
Associate Professor in Chemistry
Department of Chemistry
P.D.E.A’s.
Prof. Ramkrishna More
Arts, Commerce and Science College Akurdi, Pune-
411044

2. 05/08/2020
D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More
College, Akurdi, Pune
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.
 Geometry of molecule can be explained on the basis
of hybridization of atomic orbitals.
 Number bonds formed are equal to number of half filled orbital in an atom.
 Force of attractions is due to orbital overlap.
 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.
05/08/2020
D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More
College, Akurdi, Pune
3

4. pz orbitals of two atoms can overlap linearly to form sigma (σ) bond
(z axis is considered as internuclear axis – a convention by IUPAC)
-
-
Internuclear axis
Side-wise overlap of orbitals
Axial overlap of orbitals
++ + ++
s Orbital overlap linearly to form sigma bond
Internuclear axis
+
z
- z
P orbitals of two atoms can overlap side-wise to form Pi (π) bond
+ Internuclear axis
-
y
-
y
-
y
-
y
05/08/2020
D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More
College, Akurdi, Pune
4

5.  Electronic configuration of ‘H’ atom is 1s1
Example of VBT: Formation of H2 Molecule
 Half filled 1s orbital of one H atom overlap with Half filled 1s orbital of another H atom to form H-
H sigma bond.
 H atom consists of half filled 1s orbital which take part in bonding.
++ + ++
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.
05/08/2020
D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More
College, Akurdi, Pune
5

6.  Electronic configuration of 'O’ atom is 1s2, 2s2, 2px
2 2py
1 2pz
1
Example of VBT: Formation of O2 Molecule
 If the pz orbital of one 'O' atom overlaps with the pz orbital of other 'O' atom along
internuclear axis to form σ bond.
 py orbital and pz orbital are half filled.
 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
05/08/2020
D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More
College, Akurdi, Pune
6

7.  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.
 In in oxygen molecule double bond is formed (O=O).
One is σ bond another is π bond.
Py orbitals of two O atoms overlap side-wise manner to form Pi (π) bond
+ Internuclear axis
-
y
-
y
-
y
-
y
+
σ - bond
 Overall orbital overlapping is -
py
pz
-
py
pz
-
05/08/2020
D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More
College, Akurdi, Pune
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.
BF3
CH4
To explain structure of these molecules the concept of
hybridization is introduced in VBT.
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˚.
PCl5
05/08/2020
D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More
College, Akurdi, Pune
8

9. 05/08/2020
D. R. Shinde, Asociate Prof., P.D.E.A's. Prof Ramkrishna More
College, Akurdi, Pune
9