JEE Main · 2023 · 24 Jan 2023 · Shift 2 · Chemical Bonding and Molecular Structure

What is the number of unpaired electron(s) in the highest occupied molecular orbital of the following species :

N_{2} : N_{2}^{+}; O_{2}; O_{2}^{+}

Answer: (A)

Solution

The highest occupied molecular orbital is read off each species' configuration. In

N_{2}

the filling ends at

σ 2 p_{z}

, which holds two paired electrons, so its HOMO carries no unpaired electron. Removing one electron for

N_{2}^{+}

leaves

σ 2 p_{z}^{1}

, a single unpaired electron. For

O_{2}

the last two electrons enter the degenerate

π^{*} 2 p_{x}

and

π^{*} 2 p_{y}

orbitals, one in each, so two unpaired electrons occupy the HOMO. Taking one electron away for

O_{2}^{+}

leaves a lone electron in one

π^{*}

orbital, that is one unpaired electron. The count across the four species is

0, 1, 2, 1

JEE Main · 2023 · 24 Jan 2023 · Shift 2 · Single correct option