An ionic compound is held together by opposite charges: usually a metal (which forms a positive ion) with a nonmetal or a polyatomic ion (which forms a negative one).
Name to formula
- Write each ion with its charge: calcium Ca²⁺, chloride Cl⁻.
- Balance the charges to zero: one Ca²⁺ (+2) needs two Cl⁻ (−2).
- Write the numbers as subscripts: CaCl₂.
A shortcut many teachers use is the criss-cross: the size of each charge becomes the other ion's subscript. Al³⁺ and O²⁻ give Al₂O₃. Simplify if you can: Mg²⁺ and O²⁻ give MgO, not Mg₂O₂.
Formula to name
- Cation first, then anion: sodium chloride.
- Single-element anions end in -ide.
- No prefixes: MgCl₂ is magnesium chloride.
- If the metal can have more than one charge, add it in Roman numerals: FeCl₃ is iron(III) chloride. Work it out backward: three Cl⁻ (−3) means Fe must be +3.
Polyatomic ions
They keep their names (from Unit 1): sodium sulfate, calcium carbonate, ammonium chloride. When you need more than one, use brackets: Ca(OH)₂ has two hydroxide ions.
Watch out
-ide means one element; -ate and -ite mean oxygen is included. Potassium sulfide is K₂S; potassium sulfate is K₂SO₄.