He $(Z=2)$, So, Total electron in $\mathrm{He}_{2}=4$

Electron configuration in $\mathrm{MO} \mathrm{He}_{2}:\left(\sigma_{1 s}\right)^{2}\left(\sigma_{1 s}^{*}\right)^{2}$

All electron are paired in $\mathrm{He}_{2}$, so it is diamagnetic.

Bond order $=\frac{1}{2}\left(\mathrm{~N}_{\mathrm{b}}-\mathrm{N}_{\mathrm{a}}\right)=\frac{1}{2}(2-2)=0$

Bond order in $\mathrm{He}_{2}$ is zero. So it is unstable and does not exist.

$\mathrm{MO}$ energy diagram of $\mathrm{He}_{2}$ is as under.