The electronic configuration of oxygen atom is 1s 2 , 2s 2 , 2p 4 . Each oxygen atom has 8 electrons. Hence in each O 22– ion there are 16 + 2 = 18 electrons. The electronic configuration of O 22– ion is

KK σ (2s) 2 σ * (2s) 2 π (2p z ) 2 π (2p x ) p (2p y ) 2 π *(2p x ) 2 π * (2p y ) 2

Here KK represents non-bonding molecular orbital of 1s orbital. O 22– contains no unpaired electrons.

The electronic configuration of B 2 molecule is

KK σ (2s) 2 σ *(2s) 2 π (2p x ) 1 π (2p z ) 1

it contains 2 unpaired electrons. The electronic configuration of N 2+ ion is

KK σ (2s) 2 σ *(2s) 2 π (2p x ) 2 σ (2p y ) 2 σ (2p z ) 1

it contains one unpaired electron.

The electronic configuration of O 2 molecule is

KK σ (2s) 2 σ *(2s) 2 π (2p x ) 2 π (2p y ) 2 σ (2p z ) 2 π *(2p x ) 1 π *(2p y ) 1

It contains 2 unpaired electron.