To determine the increasing order of bond order, we apply the principles of Molecular Orbital Theory (MOT) to calculate the bond order (B.O.) for each species using the formula: B.O.=21(Nb−Na), where Nb is the number of bonding electrons and Na is the number of antibonding electrons .
- He2+: This species has 3 electrons. The configuration is (σ1s)2(σ∗1s)1 . Bond Order = 21(2−1)=0.5.
- O2− (Superoxide ion): This species has 17 electrons. Starting from the O2 configuration (16 electrons, B.O. = 2.0), the additional electron occupies an antibonding π∗ orbital . Bond Order = 21(10−7)=1.5 .
- NO (Nitric oxide): This species has 15 electrons. The configuration includes 10 bonding electrons and 5 antibonding electrons (with one electron in a π∗ orbital). Bond Order = 21(10−5)=2.5.
- C22−: This species has 14 electrons and is isoelectronic with N2 . The configuration is (σ1s)2(σ∗1s)2(σ2s)2(σ∗2s)2(π2px2=π2py2)(σ2pz)2 . Bond Order = 21(10−4)=3.0 .
Comparing the values: He2+(0.5)<O2−(1.5)<NO(2.5)<C22−(3.0). Thus, the correct increasing order is given in Option 3.