Bond order is defined as one half of the difference between the number of electrons present in the bonding and anti-bonding orbitals of a molecule.

If N_a is equal to the number of electrons in an anti-bonding orbital, then N_b is equal to the number of electrons in a bonding orbital.

Bond Order:

If N_b > N_a, then the molecule is said be stable. However, if N_b≤ N_a, then the molecule is considered to be unstable.

Bond order of N₂ :

Number of bonding electrons, N_b = 10

Number of anti-bonding electrons, N_a = 4

Bond order of nitrogen molecule= 1/2 (10-4)=3

Bond order of O₂:

Number of bonding electrons, N_b = 8

Number of anti-bonding electrons, N_a = 4.

Bond order of Oxygen molecule = 1/2 (8-4)=2

Bond order of O₂⁺ :

Number of bonding electrons, N_b = 8

Number of anti-bonding electrons, N_a = 3

Bond order of O₂^- = 1/2 (8-3) = 2.5

Bond order of O₂^-:

Number of bonding electrons, N_b = 8

Number of anti-bonding electrons, N_a = 5

Bond order of O₂^- = 1/2 (8-5) = 1.5