# Why are polyatomic ions covalent?

In the ammonium ion, $N {H}_{4}$, nitrogen is covalently bonded (shares electrons)to four separate hydrogen atoms.
In the nitrate ion, $N {O}_{3}$, there are two single covalent bonds between the central nitrogen atom and two oxygen atoms and a double covalent bond with a third oxygen atom. The valence electron number for this compound is 23, ( 5 from nitrogen , 6 from each oxygen). Because the compound contains an extra electron, the overall charge is minus one.