Question

Why is Na3N the formula for sodium nitride?

Answer 1

Because valence of nitride is 3- and valency of sodium is 1+ threfore it is Na3N

Answer 2

It s becuase the valency of Sodium(Na) is +1 and the valency of Nitrogen(N) is -3, so when they combine , the element formed is Na3N.
Three sodium atoms donate one electron each(to attain stability) to one nitrogen atom( to attain stability) as nitrogen has three electrons less in its outermost shell .