Why is Na3N the formula for sodium nitride?

Answer 1

In an ionic compound the charges must balance out to zero. Since the nitride ion has a 3- charge and sodium forms a 1+ ion so in sodium nitride there are three sodium ions (Na+) fore every one nitride ion (N3-).

Answer 2

The formula for sodium nitride is Na3N because sodium is in Group 1 of the Periodic Table with a +1 charge, and nitrogen is in Group 15 with a -3 charge. To balance the charges, three sodium atoms (each with a +1 charge) are needed to combine with one nitrogen atom (with a -3 charge), resulting in the formula Na3N.