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Oxidation number of oxygen in superoxide ion, \[{\text{O}}_{\text{2}}^ - \] is:
(A) \[ -2\]
(B) \[ -\dfrac{1}{2}\]
(C) \[ -\dfrac{3}{2}\]
(D) none of these

Answer
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Hint: In an ion, the sum of the oxidation numbers of all the atoms is equal to zero. In superoxide ion, having two oxygen atoms and a unit negative charge, the sum of the oxidation numbers of two oxygen atoms is - 1.

Complete step by step answer:
Superoxide ion is formed when an oxygen molecule gains one electron. Similarly, superoxide ions are oxide ions and peroxide ions. An oxide ion has one oxygen atom and two negative charges. A peroxide ion has two oxygen atoms and two negative charges.
The chemical formula of superoxide ion is \[{\text{O}}_{\text{2}}^ - \]. It contains two oxygen atoms and a negative charge.
Let x be the oxidation number of oxygen in superoxide ion, \[{\text{O}}_{\text{2}}^ - \].
In a neutral atom, the sum of the oxidation numbers of all the atoms is equal to zero. However, in the ion, the sum of the oxidation number of all the atoms is equal to the charge on the ion. The superoxide ion has unit negative charge. Hence, the sum of the oxidation numbers of two oxygen atoms will be equal to -1.
Calculate the oxidation number of oxygen in superoxide ion, \[{\text{O}}_{\text{2}}^ - \]
2x = - 1
x = $- \dfrac{1}{2}$
Hence, the oxidation number of oxygen in superoxide ion, \[{\text{O}}_{\text{2}}^ - \] is \[ -
\dfrac{1}{2}\] .

Thus, the correct option is the option (B).

Note: Oxidation number represents the number of electrons gained or lost to form an ion. A cation is obtained when a neutral atom loses either one or more electrons. An anion is formed when a neutral atom gains either one or more electrons.