Naming Binary Ionic Compounds
A binary ionic compound contains a metal cation and a nonmetal anion, and it is named by writing the name of the metal first, followed by the name of the nonmetal with its ending changed to “-ide”, as in sodium chloride (NaCl) and magnesium oxide (MgO). In brief: if the metal can form more than one charge, add the charge as a Roman numeral in parentheses, as in iron(III) oxide for Fe2O3; and to write the formula, use enough of each ion to make the total charge zero.
Systematic names allow chemists all over the world to identify a compound from its name. For binary ionic compounds the rules are short and consistent. This article describes the rules for naming and writing formulas, shows the common ions in tables, explains the Stock system for metals with variable charge, works through examples in both directions and tells the story of chemical nomenclature.
What are the rules for naming?
- Name the cation first. A monatomic cation has the same name as its element: sodium, calcium, aluminum.
- Name the anion second. Take the stem of the nonmetal’s name and add the ending “-ide”: chlorine becomes chloride, oxygen becomes oxide.
- Do not use prefixes such as di- or tri- to show the number of ions in an ionic compound. The ratio is fixed by the charges.
- If the metal can have more than one charge, write the charge as a Roman numeral in parentheses directly after the metal name, without a space: iron(II), iron(III).
What are the names of common monatomic anions?
| Element | Ion | Name | Charge |
|---|---|---|---|
| Hydrogen | H− | hydride | 1− |
| Fluorine | F− | fluoride | 1− |
| Chlorine | Cl− | chloride | 1− |
| Bromine | Br− | bromide | 1− |
| Iodine | I− | iodide | 1− |
| Oxygen | O2− | oxide | 2− |
| Sulfur | S2− | sulfide | 2− |
| Nitrogen | N3− | nitride | 3− |
| Phosphorus | P3− | phosphide | 3− |
Which metals have a fixed charge?
The metals of groups 1 and 2 always form 1+ and 2+ ions, aluminum forms 3+, and some others have only one common charge: zinc (2+), cadmium (2+) and silver (1+). They are named without Roman numerals. Calcium chloride is CaCl2, aluminum oxide is Al2O3 and zinc sulfide is ZnS.
How do you name compounds of metals with variable charge?
Many transition metals and some heavier main-group metals can form more than one ion. The Stock system (named after Alfred Stock) shows the charge of the cation with a Roman numeral. An older system uses the suffix “-ous” for the lower charge and “-ic” for the higher charge, with Latin stems: ferrous for iron(II) and ferric for iron(III). The Stock system is preferred, because it works for all metals.
| Metal | Ions | Stock names | Old names |
|---|---|---|---|
| Iron | Fe2+, Fe3+ | iron(II), iron(III) | ferrous, ferric |
| Copper | Cu+, Cu2+ | copper(I), copper(II) | cuprous, cupric |
| Tin | Sn2+, Sn4+ | tin(II), tin(IV) | stannous, stannic |
| Lead | Pb2+, Pb4+ | lead(II), lead(IV) | plumbous, plumbic |
| Mercury | Hg22+, Hg2+ | mercury(I), mercury(II) | mercurous, mercuric |
| Chromium | Cr2+, Cr3+ | chromium(II), chromium(III) | chromous, chromic |
| Cobalt | Co2+, Co3+ | cobalt(II), cobalt(III) | cobaltous, cobaltic |
How do you write the formula from the name?
- Write the symbols and charges of the cation and anion.
- Find the smallest numbers of each ion that make the total charge zero. The crisscross method uses the numerical value of each charge as the subscript of the other ion.
- Reduce the subscripts to the lowest whole-number ratio.
- Omit the subscript 1 and the charges in the final formula.
Examples of names and formulas
| Formula | Ions | Name |
|---|---|---|
| NaCl | Na+, Cl− | sodium chloride |
| MgBr2 | Mg2+, 2Br− | magnesium bromide |
| Al2O3 | 2Al3+, 3O2− | aluminum oxide |
| K3N | 3K+, N3− | potassium nitride |
| FeCl2 | Fe2+, 2Cl− | iron(II) chloride |
| FeCl3 | Fe3+, 3Cl− | iron(III) chloride |
| Cu2O | 2Cu+, O2− | copper(I) oxide |
| CuO | Cu2+, O2− | copper(II) oxide |
| SnCl4 | Sn4+, 4Cl− | tin(IV) chloride |
Worked examples
- Name Fe2O3. Oxide is O2−, so three oxide ions carry a charge of −6. Two iron ions must carry +6 in total, so each is +3. The name is iron(III) oxide.
- Name MnO2. Two O2− give −4, so manganese is +4. The name is manganese(IV) oxide.
- Formula of calcium nitride. Ca2+ and N3−. The crisscross gives Ca3N2. Check: 3 × (+2) = +6 and 2 × (−3) = −6. The total is zero.
- Formula of chromium(III) sulfide. Cr3+ and S2−. The crisscross gives Cr2S3, since 2 × 3 = 6 = 3 × 2.
- Reducing subscripts. Lead(IV) oxide: Pb4+ and O2−. The crisscross gives Pb2O4, which reduces to PbO2.
- Mass percent. The percentage of iron in iron(III) oxide: 2 × 55.85 = 111.70 g and 3 × 16.00 = 48.00 g of oxygen, and the total is 159.70 g/mol. The iron makes up 111.70 ÷ 159.70 × 100% = 69.94%.
How did chemical names develop?
Before the 18th century, substances had traditional names such as “oil of vitriol” and “salt of Saturn”. In 1787, Antoine Lavoisier, Louis-Bernard Guyton de Morveau, Claude Louis Berthollet and Antoine Fourcroy published the Méthode de nomenclature chimique, which proposed systematic names based on composition, and gave us names such as “sulfate” and “oxide”. The Stock system was proposed in 1919. IUPAC, founded in 1919, now maintains the international rules, in a set of publications known as the “Red Book” for inorganic chemistry.
Which mistakes are common?
Mistake 1: Using prefixes for ionic compounds
Say magnesium chloride, not magnesium dichloride.
Mistake 2: Leaving out the Roman numeral for a metal with variable charge
“Iron chloride” is ambiguous. Specify iron(II) or iron(III).
Mistake 3: Putting the Roman numeral on a fixed-charge metal
Write sodium chloride, not sodium(I) chloride, since sodium is always 1+ in compounds.
Mistake 4: Writing the charge as a subscript
Subscripts count atoms. Use the charge to find the subscripts, but do not write it in the formula.