Ncert Exercises & Solutions

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

The electronic configuration of Beryllium is

1 s^{2} 2 s^{2 .}

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

σ_{1 s}^{2} σ_{1 s}^{. 2} σ_{2 s}^{2} σ_{2 s}^{. 2}

Hence, the bond order for Be2 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
Bond order of Be2 =

\frac{1}{2} (4 - 4) =

0
A negative or zero bond order means that the molecule is unstable. Hence,Be2 molecule does not exist.