Question:
Use molecular orbital theory to explain why the Be2 molecule does not exist.
Solution:
The electronic configuration of Beryllium is $1 s^{2} 2 s^{2}$.
The molecular orbital electronic configuration for Be2 molecule can be written as:
Hence, the bond order for $\mathrm{Be}_{2}$ is $\frac{1}{2}\left(N_{b}-N_{a}\right)$.
Where,
Nb = Number of electrons in bonding orbitals
Na = Number of electrons in anti-bonding orbitals
$\therefore$ Bond order of $\mathrm{Be}_{2}=\frac{1}{2}(4-4)=0$
A negative or zero bond order means that the molecule is unstable. Hence, Be2 molecule does not exist.
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