Answer:
Two-element compounds are usually ionic when one element is a metal and the other is a non-metal.
Explanation:
sodium chloride: NaCl, with Na+ and Cl- ions
lithium nitride: Li3N, with Li+ and N3- ions
magnesium oxide: MgO, with Mg2+ and O2- ions
calcium phosphide: Ca3P2, with Ca2+ and P3- ions
