Chapter 5 · Molecules and Compounds
5.7Naming Ionic Compounds
12 min · two checks
Predict
What is the name of CuCl₂?
The idea
- Name binary ionic compounds, including metals with variable charge.
- Name compounds that contain polyatomic ions.
Name the cation first, then the anion. Monatomic anions drop the element ending and take -ide: chloride, oxide, sulfide, nitride. Fixed-charge cations need no numeral: Group 1, Group 2, Ag⁺ usually, Zn²⁺, Al³⁺. Cations that vary — most transition metals, plus lead and tin — take a roman numeral equal to the charge, deduced from the anion. FeCl₃ is iron(III) chloride because three Cl⁻ require Fe³⁺. FeCl₂ is iron(II) chloride.
Polyatomic ions keep their own names. NaNO₃ is sodium nitrate. CuSO₄ is copper(II) sulfate. NH₄Cl is ammonium chloride; ammonium is the cation even though it contains no metal. Learn this working set: OH⁻ hydroxide, NO₃⁻ nitrate, NO₂⁻ nitrite, CO₃²⁻ carbonate, SO₄²⁻ sulfate, SO₃²⁻ sulfite, PO₄³⁻ phosphate, NH₄⁺ ammonium, ClO₃⁻ chlorate, C₂H₃O₂⁻ acetate. The -ate ion has one more oxygen than the -ite ion and the same charge.
Keep these
- Cation then anion. Monatomic anion ends in -ide.
- Roman numeral = cation charge, not the subscript.
- Polyatomic names are memorized; -ate has more oxygen than -ite.
Worked path
Name Cr(NO₃)₃ and write the formula for cobalt(II) phosphate. Phosphate is PO₄³⁻.
Observe
Nitrate is 1−. Three nitrates mean chromium is 3+. Cobalt(II) is Co²⁺.