Question

What is the oxidation number of underlined element?

${\underset{–}{S}}^{2-}$

Answer 1

Let the oxidation number of $S$ in $S^{2-}$ be $x$.
The oxidation number of a monoatomic ion is equal to its charge.
Therefore, $x=-2$
Hence, the oxidation number of $S$ in $S^{2-}$ is $-2$.