Engineering Chemistry S Datta

VALENCY AND CHEMICAL BONDING 29
(b) Side-by-side overlap (Fig. 2.24):
E
+
+
+
+ –
2 *2p
– +
A (Antibonding)
– –
+
–
2 2p Bonding
Fig. 2.24 π-Bonding.
Linear combination of two 2p z
orbitals to form π z
2p (bonding) and π z
2p* (antibonding)
molecular orbitals (π-symmetry).
* 2p
* y 2p, * z 2p
2p
z
y
z
z y x
* y
2p, y
2p
2p
Energy
* 2s
2s
2s
2s
Fig. 2.25 Order of energy of molecular orbitals (M.O.).
(Energy level diagram of molecular orbitals for a homonuclear diatomic molecule)
So, the order of energy for molecular orbital is:
σ 1s
< σ 1s
* < σ 2s
< σ 2s
* < σ 2p
< π y 2 p
< π y∗2
p
< σ
π z2
p π z∗2
p
2p
Bond-Order (B.O.):
Bond-order (B.O.) of a molecule is calculated as follows:
B.O. ⇒
N.B. − N.A.
2
N.B. ⇒ no. of electrons in bonding molecular orbitals.
N.A. ⇒ no. of electrons in antibonding molecular orbitals.