$\text { B.O. }=\frac{8-4}{2}=2$

$\mathrm{O}_2$

$\left(16 \mathrm{e}^{-}\right): \sigma 1 \mathrm{~s}^2, \sigma^* 1 \mathrm{~s}^2, \sigma 2 \mathrm{~s}^2, \sigma^* 2 \mathrm{~s}^2, \sigma 2 \mathrm{pz}^2$

${\left[\pi 2 \mathrm{p}_{\mathrm{x}}^2=\pi 2 \mathrm{p}_{\mathrm{y}}^2\right]\left[\pi^* 2 \mathrm{p}_{\mathrm{x}}^1=\pi^* 2 \mathrm{p}_{\mathrm{y}}^1\right]}$

$\text { B.O. }=\frac{10-6}{2}=2$

$\mathrm{Be}_2$

$\left(8 \mathrm{e}^{-}\right): \sigma 1 \mathrm{~s}^2, \sigma^* 1 \mathrm{~s}^2, \sigma 2 \mathrm{~s}^2, \sigma^* 2 \mathrm{~s}^2$

$\text { B.O. }=\frac{4-4}{2}=0$

$\text { Li }_2$

$\left(6 \mathrm{e}^{-}\right): \sigma 1 \mathrm{~s}^2, \sigma^* 1 \mathrm{~s}^2, \sigma 2 \mathrm{~s}^2$