Chapter 5 · Molecules and Compounds
5.5Writing Formulas for Ionic Compounds
12 min · two checks
Predict
What is the formula of the compound formed by Al³⁺ and O²⁻?
The idea
- Write ionic formulas that are charge-neutral.
- Use parentheses for more than one polyatomic ion.
Ionic compounds are neutral overall. Find the smallest whole numbers of each ion whose charges cancel. A reliable path: write the ions with charges, then use the magnitude of each charge as the other ion’s subscript, and reduce. Ca²⁺ and Cl⁻ give CaCl₂. Mg²⁺ and O²⁻ give MgO, not Mg₂O₂, because you reduce by 2. Al³⁺ and O²⁻ give Al₂O₃, which does not reduce.
Polyatomic ions are units. If you need more than one, wrap the ion in parentheses before the subscript: calcium nitrate is Ca(NO₃)₂, not CaNO₃₂. Never include the charges in the finished formula. The formula states the ratio only. If the name contains a roman numeral, that numeral is the cation charge, not a subscript: iron(III) oxide uses Fe³⁺, so the formula is Fe₂O₃.
Keep these
- Total positive charge equals total negative charge.
- Reduce subscripts to the smallest integers.
- Parentheses go around polyatomic ions when the subscript is greater than 1.
Worked path
Write the formula for iron(III) sulfate. Sulfate is SO₄²⁻.
Observe
Iron(III) means Fe³⁺. Sulfate is SO₄²⁻.