Question

Oxidation number of oxygen in $O_2$ molecule is
(A) +1
(B) 0
(C) +2
(D) -2

Answer 1

Hint: To solve this question, we first need to know what the oxidation number is. In a chemical compound, the oxidation number or the oxidation state is used to determine the degree of oxidation of an atom.

Complete answer:
The rules to determine the oxidation number of an element are as follows:
- For a free element, the oxidation number of an element in a is always 0.
For example, in helium and nitrogen, the oxidation number of the He and N atom in helium gas and nitrogen ($N_2$) gas will be 0.
Similarly, the sum of all the atom's oxidation numbers of a compound that is neutral is 0.
- The oxidation number of an ion, whether it is monatomic or polyatomic, is equal to the charge on the in.
For example, the oxidation number of $M{g}^{2+}$ is +2. The oxidation number of phosphate ions $P{O_4}^{3-}$ is -3.
- The usual oxidation number of
Hydrogen = +1
Oxygen = -2
Group I(A) = +1
Group II(A) = +2
Group VII(A) = -1
Now let us look at oxygen gas ($O_2$).
Here, both the oxygen atoms have the same electronegativities. Since oxygen gas i.e., diatomic oxygen exists in the atmosphere in the free state, hence the oxidation state of each atom will be 0.

So, the correct option is (B) 0 .

Note:
It should be noted that there are some exceptions while assigning the oxidation states of atoms in a molecule.
- When a hydrogen atom is bonded to a less electronegative atom, its oxidation number is -1.
- When an oxygen atom is bonded to a more electronegative atom, it exists in a peroxide ion, its oxidation number changes.
- When a Group VII(A) atom is bonded to a more electronegative atom, its oxidation number changes.