Overview
This lecture reviews how to name ionic compounds and write their formulas based on constituent ions, including recognizing cations, anions, and polyatomic ions.
When to Use Ionic Naming
- Use ionic naming when you have a metal or ammonium ion combined with a nonmetal or polyatomic ion.
- Cations (positively charged ions) are written and named first; anions (negatively charged ions) are second.
Naming Cations and Anions
- Cations keep their elemental name unless they have more than one possible charge.
- Transition metals with multiple possible charges use Roman numerals to show their charge (e.g., iron(II)).
- The older system uses "-ic" for the higher charge and "-ous" for the lower charge (rarely required).
- Anions drop the elemental name's ending and add "-ide" (e.g., chloride from chlorine).
Naming Examples
- Potassium bromide: Potassium is a group 1 metal, so name stays as is; bromine becomes bromide.
- Sodium nitrate: Sodium keeps its name; nitrate (polyatomic ion) keeps its name.
- Iron(II) chloride: Iron is a transition metal; use Roman numeral for its +2 charge; chloride is the anion.
- Calcium oxide: Calcium (group 2) keeps its name; oxygen becomes oxide.
- Manganese(III) sulfide: Manganese is transition metal; use Roman numeral for +3; sulfur becomes sulfide.
- Lithium oxide: Lithium (group 1) keeps its name; oxygen becomes oxide.
- Magnesium bromide: Magnesium keeps its name; bromine becomes bromide.
- Cadmium(II) nitrate: Cadmium with +2 charge; nitrate is polyatomic and keeps its name.
Writing Formulas from Names
- Use ion charges to determine subscripts that balance total charges to zero.
- For polyatomic ions needing multiple copies, use parentheses around the ion.
- Example: Chromium(III) carbonate → Cr₂(CO₃)₃ (Cr³⁺ and CO₃²⁻).
- Potassium iodide: K⁺ and I⁻ form KI.
- Iron(II) oxide: Fe²⁺ and O²⁻ form FeO.
- Sodium nitrite: Na⁺ and NO₂⁻ form NaNO₂.
- Nickel(II) sulfate: Ni²⁺ and SO₄²⁻ form NiSO₄.
Key Terms & Definitions
- Cation — positively charged ion (usually a metal or ammonium).
- Anion — negatively charged ion (nonmetal or polyatomic ion).
- Polyatomic ion — an ion composed of more than one atom (e.g., NO₃⁻, SO₄²⁻).
- Transition metal — elements in the d-block of the periodic table, often with multiple possible charges.
- Roman numeral — notation used to indicate the charge of transition metals in compound names.
Action Items / Next Steps
- Practice naming ionic compounds and writing formulas for given pairs of ions.
- Memorize common polyatomic ions and their charges for use in naming and formula writing.