$He_{2}$ molecule is not possible. Explain.

(N/A) $He$ $(Z=2)$,so the total number of electrons in $He_{2}$ is $4$.
The molecular orbital $(MO)$ electronic configuration of $He_{2}$ is: $(\sigma_{1s})^{2} (\sigma_{1s}^{*})^{2}$.
All electrons are paired in $He_{2}$,so it is diamagnetic.
Bond order $= \frac{1}{2} (N_{b} - N_{a}) = \frac{1}{2} (2 - 2) = 0$.
Since the bond order of $He_{2}$ is $0$,the molecule is unstable and does not exist.
The $MO$ energy diagram of $He_{2}$ is shown below.