How can I write the formula for sodium phosphide?

Sodium Phosphide has a formula of ##Na_3P##.

Sodium is a metal cation with a charge of ##Na^(+1)## Phosphorus is a nonmetal anion with a charge of ##P^(-3)##

In order to bond ionically the charges must be equal and opposite. It will take one -3 phosphide ion to balance three +1 sodium ions forming a magnesium phosphide molecule of ##Na_3P##.

I hope this was helpful. SMARTERTEACHER