Explanation

(a)
$O_{2}^{-} = 8 + 8 + 1 = \sigma 1s^{2},\ _{\sigma}^{*}1s^{2,}\sigma 2s^{2},\ _{\sigma}^{*}2s^{2,}\ \sigma 2p_{z}^{2,}\ \pi 2p_{x}^{2} = \pi 2p_{y\ }^{2},\ _{\pi}^{*}2p_{x}^{2} = \ _{\pi}^{*}2p_{y}^{1}$
$\therefore$ Total antibonding electrons = 7
$O_{2} = 8 + 8 = 16$
$\ = \sigma 1s^{2},\ _{\sigma}^{*}1s^{2}\ ,\sigma 2s^{2},\ _{\sigma}^{*}2s^{2,}\sigma 2p_{z}^{2}\ ,\ \pi 2p_{x}^{2} = \pi 2p_{y\ }^{2},\pi 2p_{x}^{1} = \ _{\pi}^{*}2p_{y}^{1}$
$\therefore$ Total antibonding electrons = 6
$O_{2}^{2 -} = 8 + 8 + 2 = 18$
$\ \ \ \ \ \ \ \ \ \ \ = \sigma 1s^{2,}\ _{\sigma}^{*}1s^{2,}\ \sigma 2s^{2,}\ _{\sigma}^{*}2s^{2,}\sigma 2p_{z}^{2}\ ,\ \pi 2p_{x}^{2} = \pi 2p_{y\ }^{2},\ _{\pi}^{*}2p_{x}^{2} = \ _{\pi}^{*}2p_{y}^{2}$
$\therefore$ Total antibonding electrons = 8