Question

Use molecular orbital theory to explain why the $Be_2$ molecule does not exist.

Answer 1

The electronic configuration of Beryllium is $1s^2\; 2s^2$.

The molecular orbital electronic configuration for $Be_2$ molecule can be written as:
$\sigma^2_{1s} \sigma^{*2}_{1s} \sigma^2_{2s} \sigma^{*2}_{2s}$
Hence, the bond order for $Be_2$ is $\frac{1}{2}(N_b-N_a)$
Where,
$N_b$ = Number of electrons in bonding orbitals
$N_a$ = Number of electrons in anti-bonding orbitals
$\therefore$ Bond order of $Be_2$ =$\frac{1}{2}(4-4)$ = $0$
A negative or zero bond order means that the molecule is unstable.

Hence, $Be_2$ molecule does not exist.