Question

What do you mean by term bond order? Calculate the bond order of: N₂, C₂, H₂, N₂.

Answer 1

Bond order is defined as one half the difference between the number of electrons present in the bonding and the anti-bonding orbitals.
Bond order (B.O.) = 1/2 (N_b – N_a)
where, N_b and N_a are number of electrons in bonding and anti bonding molecular orbitals respectively. Integral bond order values of 1, 2 or 3 correspond to single, double or triple bonds respectively.

A positive bond order (i.e., N_b > N_a) means a stable molecule while a negative (i.e., N_b < N_a) or zero (i.e., N_b = N_a) bond order means an unstable molecule.

(a) Bond order of N₂
Molecular orbital electronic configuration of N₂

Number of bonding electrons = 10
Number of anti-bonding electrons = 4
Bond order of nitrogen molecule = 1/2 (10 – 4) = 3

(b) Bond order of C₂
Total number of electrons in C₂ molecule is 6 + 6 = 12.
Molecular orbital electronic configuration of C₂:

The bond order is 2 and so carbon molecule will have a double bond.

(c) Bond order of H^-₂
Total number of electrons = 2 + 1 = 3
Molecular orbital electron is configuration

(d) Bond order of N^-₂
Total number of electrons = 14 + 1 = 15
Molecular orbital electronic configuration

Electrons in bonding orbitals : 10
Electrons in anti bonding orbitals : 5
Bond order of N^-₂ = 1/2(10 – 5) = 2.5