How to Write Ionic Compound Formulas

1. Write the ion charges

Example: magnesium is Mg2+ and oxygen is O2-.

2. Balance total charge to zero

Mg2+ and O2- balance in a 1:1 ratio, so the formula is MgO.

3. Use parentheses for polyatomic ions

Calcium nitrate is Ca(NO3)2 because two nitrate ions are needed to balance Ca2+.