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Chemical names can look intimidating — “iron(III) sulfate”, “ammonium dichromate”, “copper(II) nitrate pentahydrate” — but ionic compound names follow a small set of rules that almost never change. Learn them once and you can name thousands of compounds, and write their formulas, without memorizing each one.
The basic pattern
An ionic compound is made of a positive ion (cation) and a negative ion (anion). The name is simply:
cation name + anion name
The cation always comes first, exactly as in the formula.
Rule 1: metal cations keep the element’s name
Na⁺ is sodium, Ca²⁺ is calcium, Al³⁺ is aluminium. Nothing changes.
Rule 2: simple anions end in “-ide”
A single-element anion takes the root of the element’s name plus -ide:
| Element | Anion | Name |
|---|---|---|
| Fluorine | F⁻ | fluoride |
| Chlorine | Cl⁻ | chloride |
| Bromine | Br⁻ | bromide |
| Iodine | I⁻ | iodide |
| Oxygen | O²⁻ | oxide |
| Sulfur | S²⁻ | sulfide |
| Nitrogen | N³⁻ | nitride |
| Phosphorus | P³⁻ | phosphide |
So NaCl is sodium chloride, MgO is magnesium oxide, Al₂S₃ is aluminium sulfide.
Notice: no prefixes. Ionic compounds are not named “dialuminium trisulfide”. The charges fix the ratio, so the numbers are implied. (Prefixes like di- and tri- are for covalent compounds.)
Rule 3: Roman numerals for metals with more than one charge
Many transition metals — and a few others like tin and lead — can form ions with different charges. Iron can be Fe²⁺ or Fe³⁺. The name must say which, using a Roman numeral in brackets:
- FeCl₂ → iron(II) chloride
- FeCl₃ → iron(III) chloride
- Cu₂O → copper(I) oxide
- CuO → copper(II) oxide
- PbO₂ → lead(IV) oxide
The numeral is the ion’s charge, not the number of atoms. To find it, work backwards from the anion. In Fe₂O₃, three oxide ions give −6, so two irons must give +6: each is Fe³⁺. It’s iron(III) oxide. This is exactly the method for finding oxidation numbers.
Metals with only one common charge don’t get a numeral: group 1 (always +1), group 2 (always +2), aluminium (+3), zinc (+2) and silver (+1). Writing “sodium(I) chloride” isn’t wrong, just unnecessary.
You’ll still see an older system in some places: -ous for the lower charge and -ic for the higher, using Latin roots. Ferrous (Fe²⁺) and ferric (Fe³⁺), cuprous (Cu⁺) and cupric (Cu²⁺). Modern names use Roman numerals.
Rule 4: polyatomic ions keep their own names
Some ions are groups of atoms that stay together. Their names just have to be learned — see our polyatomic ions list. The most common:
| Ion | Name |
|---|---|
| NH₄⁺ | ammonium |
| OH⁻ | hydroxide |
| NO₃⁻ | nitrate |
| NO₂⁻ | nitrite |
| SO₄²⁻ | sulfate |
| SO₃²⁻ | sulfite |
| CO₃²⁻ | carbonate |
| HCO₃⁻ | hydrogen carbonate (bicarbonate) |
| PO₄³⁻ | phosphate |
| CH₃COO⁻ | acetate (ethanoate) |
Patterns help: -ate and -ite ions contain oxygen, and the -ite version has one fewer oxygen than the -ate version with the same charge.
Examples: NaOH is sodium hydroxide; CaCO₃ is calcium carbonate; (NH₄)₂SO₄ is ammonium sulfate; Fe(NO₃)₃ is iron(III) nitrate.
From name to formula
Go the other way by balancing the charges so they add to zero.
Aluminium oxide: Al³⁺ and O²⁻. The lowest common multiple of 3 and 2 is 6: two Al³⁺ (+6) and three O²⁻ (−6). Formula: Al₂O₃.
A quick shortcut is the criss-cross method: each ion’s charge number becomes the other ion’s subscript. Al³⁺ O²⁻ → Al₂O₃. Always reduce to the simplest ratio afterwards: Mg²⁺ O²⁻ would criss-cross to Mg₂O₂, which reduces to MgO.
Calcium phosphate: Ca²⁺ and PO₄³⁻ → Ca₃(PO₄)₂. Use brackets whenever you need more than one of a polyatomic ion.
Hydrates
For compounds with water of crystallization, add a Greek prefix plus “hydrate”: CuSO₄·5H₂O is copper(II) sulfate pentahydrate. More in hydrates explained.
Common mistakes
- Using prefixes for ionic compounds. CaCl₂ is calcium chloride, not calcium dichloride.
- Reading the Roman numeral as a count. Iron(III) oxide has two irons, not three.
- Forgetting brackets. Calcium hydroxide is Ca(OH)₂, not CaOH₂.
- Mixing up -ate and -ite. Sodium sulfate (Na₂SO₄) and sodium sulfite (Na₂SO₃) are different substances.
Quick answers
How do I know if a compound is ionic? Usually, a metal combined with a nonmetal, or any compound containing ammonium or another polyatomic ion. See ionic vs. covalent bonds.
Why does zinc not need a Roman numeral? Because it only forms Zn²⁺ in its common compounds.
Is “aluminum” or “aluminium” correct? Both. IUPAC’s name is aluminium; “aluminum” is standard in the United States.
Check a formula
Once you’ve written a formula, the molar mass calculator will confirm it parses correctly, and the oxidation number calculator tells you which Roman numeral a transition metal needs.
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