Question

What is the oxidation number of $\mathrm{O}$ in the ion ${\mathrm{OH}}^{-}$?

Answer 1

Oxidation number: Oxidation number denotes the oxidation state of an element in a compound ascertained according to a set of rules formulated on the basis that electron pair in a covalent bond belongs entirely to a more electronegative element.

Calculating the oxidation number of oxygen($\mathrm{O}$) in hydroxide ion(${\mathrm{OH}}^{-}$)

The oxidation state of Hydrogen(H) = +1

The charge on hydroxide ion(OH^-) = -1

Consider the oxidation state of Oxygen(O) = x

Now,

$$\begin{gathered} \mathrm{x}+1=-1 \\ \mathrm{x}=-2 \end{gathered}$$

Hence, the oxidation number of $\mathrm{O}$ in the ion ${\mathrm{OH}}^{-}$ is $-2$.