$\mathrm{Li}(\mathrm{Z}=3)$, So, Total electron in $\mathrm{Li}_{2}=16$

Electron configuration in $\mathrm{MO}$ for $\mathrm{Li}_{2}$ :

$(\sigma 1 s)^{2}\left(\sigma^{*} 1 s\right)^{2}(\sigma 2 s)^{2}\left(\sigma^{*} 2 s\right)^{0}$ OR KK $(\sigma 2 s)^{2}$

Magnetic Property : All electrons are paired. So, it should be diamagnetic

Where, $\mathrm{N}_{\mathrm{b}}=4, \mathrm{~N}_{\mathrm{a}}=2$

Bond order $=\frac{1}{2}\left(\mathrm{~N}_{\mathrm{b}}-\mathrm{N}_{\mathrm{a}}\right)$

$=\frac{1}{2}(4-2)=1$

So, single bond and $\mathrm{Li}_{2}$ is stable.

Note : KK means $\left[\mathrm{He}_{2}\right]=(\sigma 1 s)^{2}\left(\sigma^{*} 1 s\right)^{2}$

Energy diagram for $\mathrm{Li}_{2}$ molecule is as under.

$\mathbf{L i}_{2}: \operatorname{KK}\left(\sigma_{2 s}\right)^{2}\left(\sigma_{2 s}^{*}\right)^{0}$