Question

Use molecular orbital theory to explain why the Be₂ molecule does not exist.

Answer 1

The electronic configuration of Beryllium is ${\displaystyle 1{\text{s}}^2 2{\text{s}}^2}$.

The molecular orbital electronic configuration for Be₂ molecule can be written as:

${\displaystyle {\sigma }_{1\text{s}}^2 {\sigma }_{1\text{s}}^{•2} {\sigma }_{2\text{s}}^2 {\sigma }_{2\text{s}}^{•2}}$

Hence, the bond order for Be₂ is ${\displaystyle \frac{1}{2}({\text{N}}_{\text{b}}-{\text{N}}_{\text{a}})}$

Where,

N_b = Number of electrons in bonding orbitals

N_a = Number of electrons in anti-bonding orbitals

∴Bond order of Be₂ = ${\displaystyle \frac{1}{2}(4-4)=0}$

A negative or zero bond order means that the molecule is unstable. Hence, Be₂molecule does not exist.