Question

Unpaired electrons are present in:
A.${\text{Ba}}{{\text{O}}_2}$
B.${\text{K}}{{\text{O}}_2}$
C.${{\text{K}}_{\text{2}}}{\text{O}}$
D.${\text{N}}{{\text{a}}_{\text{2}}}{{\text{O}}_{\text{2}}}$

Answer 1

To solve this question, you must recall the electronic configurations of the central atoms in these molecules and the molecular orbital diagrams. If the species has unpaired electrons present, it is known as diamagnetic.

Complete step by step answer:
The peroxide ion has a single oxygen-oxygen covalent bond and the oxygen atoms are present in an oxidation state of $- 1$. It has five completely filled bonding molecular orbitals and four completely filled anti bonding molecular orbitals. As a result, all the electrons are paired in a peroxide ion.The superoxide ion has the chemical formula $O_2^ -$. The oxidation state of both the oxygen atoms is $- \dfrac{1}{2}$. It is a di-radical that contains two unpaired electrons in its anti-bonding pi orbitals. Thus, it is paramagnetic.
In option A, the given compound is ${\text{Ba}}{{\text{O}}_2}$ (Barium peroxide)
The cation is barium ion and the anion present is peroxide ion, which is $O_2^{2 - }$. We know that peroxide ions have completely filled molecular orbitals and all the electrons are paired.
In option B, the given compound is ${\text{K}}{{\text{O}}_2}$ (potassium superoxide)
The cation is potassium ion and the anion present is superoxide ion, which is $O_2^ -$. We know that superoxide ions have two unpaired electrons present.
In option C, the given compound is ${{\text{K}}_{\text{2}}}{\text{O}}$ (potassium peroxide).
The cation is potassium ion and the anion present is peroxide ion, which is $O_2^{2 - }$. We know that peroxide ions have completely filled molecular orbitals and all the electrons are paired.
In option D, the given compound is ${\text{N}}{{\text{a}}_{\text{2}}}{{\text{O}}_{\text{2}}}$ (sodium peroxide)
The cation is sodium ion and the anion present is peroxide ion, which is $O_2^{2 - }$. We know that peroxide ions have completely filled molecular orbitals and all the electrons are paired.

Thus, the correct option is B.

Note:
Molecular orbital theory (MOT) is a method of depicting the electronic structures of molecules using quantum mechanics. In this theory, electrons in a molecule are not conventionally assigned to individual chemical bonds between atoms, but are treated as moving under the influence of the atomic nuclei in the whole molecule.