Naming ionic compounds
Name the cation first, then the anion.
- The cation keeps the metal’s name: Na+ is sodium.
- A single-atom anion takes the ending -ide: Cl− is chloride, O2− is oxide, S2− is sulfide and N3− is nitride.
No prefixes are used. The charges already fix the ratio.
A polyatomic ion is a charged group of bonded atoms that acts as a unit.
| Ion | Name |
|---|---|
| NH4+ | ammonium |
| OH− | hydroxide |
| NO3− | nitrate |
| SO42− | sulfate |
| CO32− | carbonate |
| PO43− | phosphate |
Many transition metals form more than one ion. A Roman numeral gives the charge: Fe2+ is iron(II) and Fe3+ is iron(III).
Worked example: name FeCl3
Each chloride is 1-, and there are three, for a total of 3-. The single iron must be 3+.
iron(III) chloride
Worked example: name Ca(NO3)2
Calcium forms only a 2+ ion, so no Roman numeral is needed. NO3− is nitrate.
calcium nitrate
Common mistake
Using prefixes such as “di” or “tri”. CaCl2 is calcium chloride, not calcium dichloride.