## Chemistry: The Molecular Science (5th Edition)

In a polyatomic ion, the sum of the oxidation numbers of all atoms is equal to the overall charge on the ion. The overall charge on $NO_{3}^{-}$ ion is -1. Let the oxidation number of nitrogen be $x$. The oxidation number of each oxygen atom is -2. Therefore, we can write $x+3(-2)=-1$ Or $x=+5$