Question

Mention any two reasons for the anomalous behavior of oxygen.

Answer 1

You can start by discussing the anomalous behavior of oxygen in brief like how it exists as a gas, about it exists in diatomic form, how it can have -1 oxidation state in compounds etc. Then explain the gaseous state of oxygen by stating that the van der Waal forces between ${O_2}$ is less than between ${S_8}$ molecules (or any other molecule of group-16 elements) and explain the -1 oxidation state of oxygen by discussing how it is the second most electronegative atom.

Complete step by step answer:
The electronic configuration of Oxygen is $1{s^2}2{s^2}2{p^4}$ , also written as $\left[ {He} \right]2{s^2}2{p^4}$ .
Oxygen is group 16 of the periodic table but unlike all the other elements of group-16 oxygen exists in diatomic form instead of polyatomic form. Oxygen is gas while the others are solid. Oxygen is always paramagnetic while others are diamagnetic. Oxygen also shows a -1 oxidation state while no other element in group-16 does that

The two reasons for the anomalous behavior of oxygen are:
Physical state – As exists earlier only oxygen exists in gas form and all the other elements of group-16 exist in solid form. Oxygen forms a $p\pi - p\pi$ bond and thus it exists as a ${O_2}$ molecule. It exists in gaseous form because the van der Waal forces (the force of attraction between molecules) between ${O_2}$ molecules is very weak, whereas the other atoms, for example, say sulfur exists in the form of ${S_8}$ molecules and the van der Waal force between these ${S_8}$ molecules are relatively larger and hence, they exist in solid form.

Electronegativity – We know that oxygen is the second most electronegative element in the periodic table, (first being Fluorine). So being the second most electronegative element, oxygen shows an oxidation state of -2 in most of its compounds. Although some other elements of group 16 also show -2 oxidation state, i.e. Sulfur and Selenium. But, what makes oxygen unique is that due to its high electronegativity it can also have a -1 oxidation state in compounds like ${H_2}O$ and $N{a_2}{O_2}$ , while the other elements of group-16 elements cannot have a -1 oxidation state.

Note: Oxygen is the first element of group 16, i.e., oxygen family, of p-block in the periodic table. It is a colorless and odorless gaseous element constituting about one-fifth of the volume of the atmosphere. Its symbol is ‘O’, atomic numbers are 8 and atomic mass is 15.999amu.