Here's why:

* Nitrate ion formula: NO₃⁻

* Nitrogen (N): Has a charge of +5

* Oxygen (O): Has a charge of -2 (each oxygen atom)

* Total charge of oxygen: 3 O atoms * -2 charge/atom = -6

* Overall charge: +5 (nitrogen) - 6 (oxygen) = -1

Therefore, the nitrate ion (NO₃⁻) has a charge of -1.