Here's how to determine the formula for sodium nitride:

* Identify the ions:

* Sodium (Na) forms a +1 cation (Na⁺).

* Nitrogen (N) forms a -3 anion (N⁻³).

* Balance the charges: To achieve a neutral compound, you need three sodium ions (3 x +1 = +3) to balance the charge of one nitride ion (-3).

* Write the formula: The formula is Na₃N.