Use molecular orbital theory to explain why the $Be_{2}$ molecule does not exist.

(N/A) The atomic number of $Be$ is $4$,so its electronic configuration is $1s^{2} 2s^{2}$.
In a $Be_{2}$ molecule,there are a total of $8$ electrons.
The molecular orbital configuration of $Be_{2}$ is $(\sigma_{1s})^{2} (\sigma_{1s}^{*})^{2} (\sigma_{2s})^{2} (\sigma_{2s}^{*})^{2}$.
Here,the number of bonding electrons $(N_{b})$ is $4$ and the number of antibonding electrons $(N_{a})$ is $4$.
The bond order $(BO)$ is calculated as: $BO = \frac{1}{2}(N_{b} - N_{a}) = \frac{1}{2}(4 - 4) = 0$.
Since the bond order of $Be_{2}$ is $0$,the molecule is unstable and does not exist.