Question

Identify a molecule which does not exist.
(NEET 2020)

• A. $C_2$
• B. $O_2$
• C. $H{e}_2$
• D. $L{i}_2$

Answer 1

The correct option is C $H{e}_2$

Diatomic molecule for which bond order is zero or negative does not exist.

According to Linear combination of atomic orbitals the $H{e}_2$ molecule configuration is $\left({\sigma }_{1s}{)}^2\right({\sigma }_{1s}^{\ast }{)}^2$

Thus, there are equal number of electrons present in bonding and antibonding molecular orbitals.

Thus, $\text{Bond order}=\frac{1}{2}[N_b-N_a]$
where, $N_b$ and $N_a$ are number of electrons in bonding and antibonding molecular orbitals respectively.

Hence,

Molecule	$\text{Bond order}$
$C_2$	$2$
$O_2$	$2$
$H{e}_2$	$0$
$L{i}_2$	$1$

So, molecule $H{e}_2$ does not exist.
Hence, option (C) is the correct answer.