JEE Main · 2021 · 25 Feb 2021 · Shift 1 · Chemical Bonding and Molecular Structure

According to molecular orbital theory, the species among the following that does not exist is :

Answer: (C)

Solution

A species holds together only if its bond order, half the excess of bonding over antibonding electrons, is greater than zero.

H e_{2}^{+}

has 3 electrons,

σ_{1 s}^{2} σ_{1 s}^{* 1}

, bond order

\frac{1}{2} (2 - 1) = 0.5

H e_{2}^{-}

has 5 electrons,

σ_{1 s}^{2} σ_{1 s}^{* 2} σ_{2 s}^{1}

, bond order

\frac{1}{2} (3 - 2) = 0.5

O_{2}^{2 -}

has 18 electrons, ten bonding and eight antibonding, bond order 1.

B e_{2}

has 8 electrons,

σ_{1 s}^{2} σ_{1 s}^{* 2} σ_{2 s}^{2} σ_{2 s}^{* 2}

, so the bonding and antibonding contributions cancel exactly and the bond order is zero.

B e_{2}

therefore does not exist.

JEE Main · 2021 · 25 Feb 2021 · Shift 1 · Single correct option