HINT: $B . O .$ $=$ $N_{b}$ $-$ $N_{a} 2$

Explanation:

1. Electronic configuration of O₂ (16 electrons) $= σ_{1 s^{2}},$ $σ_{1 s^{2}}^{*},$ $σ_{2 s^{2}},$ $σ_{2 s^{2}}^{*},$ $σ_{2 {p_{z}}^{2}}, π_{2 {p_{x}}^{2}} \approx π_{2 {p_{y}}^{2}},$ $π_{2 {p_{x}}^{1}}^{*} \approx π_{2 {p_{y}}^{1}}^{*}$

Bond order

= \frac{1}{2} (N_{b} - N_{a}) = \frac{1}{2} (10 - 6) = 2

2. Electronic configuration of

O_{2}^{+}

(15 electrons)

= σ_{1 s^{2}},

σ_{1 s^{2}}^{*},

σ_{2 s^{2}},

σ_{2 s^{2}}^{*},

σ_{2 {p_{z}}^{2}}, π_{2 {p_{x}}^{2}} \approx π_{2 {p_{y}}^{2}},

π_{2 {p_{x}}^{1}}^{*} \approx π_{2 {p_{y}}^{0}}^{*}

Bond order

= \frac{1}{2} (N_{b} - N_{a}) = \frac{1}{2} (10 - 5) = 2.5

3. Electronic configuration of

O_{2}^{-}

(17 electrons)

= σ_{1 s^{2}},

σ_{1 s^{2}}^{*},

σ_{2 s^{2}},

σ_{2 s^{2}}^{*},

σ_{2 {p_{z}}^{2}}, π_{2 {p_{x}}^{2}} \approx π_{2 {p_{y}}^{2}},

π_{2 {p_{x}}^{2}}^{*} \approx π_{2 {p_{y}}^{1}}^{*}

Bond order

= \frac{1}{2} (N_{b} - N_{a}) = \frac{1}{2} (10 - 7) = 1.5

4. Electronic configuration of

O_{2}^{2 -}

(18 electrons)

= σ_{1 s^{2}},

σ_{1 s^{2}}^{*},

σ_{2 s^{2}},

σ_{2 s^{2}}^{*},

σ_{2 {p_{z}}^{2}}, π_{2 {p_{x}}^{2}} \approx π_{2 {p_{y}}^{2}},

π_{2 {p_{x}}^{2}}^{*} \approx π_{2 {p_{y}}^{2}}^{*}

Bond order

= \frac{1}{2} (N_{b} - N_{a}) = \frac{1}{2} (10 - 8) = 1

Thus, $O_{2}^{-}$ shows the bond order 1.5

AIPMT 2012