Question

What is the oxidation number of underlined element?

${\underset{–}{Mg}}_3{N}_2$

Answer 1

Let $x$ be the oxidation number of Mg in $\underset{–}{M{g}_3}{N}_2$.
Since the overall charge on the compound is $0$, the sum of oxidation states of all elements in it should be equal to $0$.
Therefore, $3x+2(-3)=0$
or, $x=+2$
Hence, the oxidation number of Mg in ${\underset{–}{Mg}}_3{N}_2$ is $+2$.