Here's why:

* Nitrogen (N) is in group 15 of the periodic table, meaning it typically forms three bonds.

* Oxygen (O) is in group 16, meaning it typically forms two bonds.

* To achieve a stable octet, nitrogen shares its three valence electrons with three oxygen atoms.

* One of the oxygen atoms forms a double bond with nitrogen, while the other two form single bonds.

* The nitrogen atom has a formal charge of +1, and each of the single-bonded oxygen atoms has a formal charge of -1.

* The overall charge of the nitrate ion is therefore -1.