Naming chemical compounds follows systematic rules that let chemists deduce a substance's composition from its name alone. Ionic compounds take the metal name first, then the non-metal with its ending changed to -ide. Covalent molecular compounds use Greek prefixes to show how many atoms of each element are present.
What is the -ide naming rule for binary ionic compounds?
A binary compound contains exactly two elements. The naming rule for binary ionic compounds is:
- Name the metal (or ammonium, NH₄⁺) first — unchanged from the element name.
- Name the non-metal second — change its ending to -ide.
| Element | Name as anion (−ide ending) |
|---|---|
| Chlorine (Cl) | Chloride |
| Oxygen (O) | Oxide |
| Sulfur (S) | Sulfide |
| Bromine (Br) | Bromide |
| Iodine (I) | Iodide |
| Nitrogen (N) | Nitride |
So: sodium + chlorine → sodium chloride (NaCl); calcium + oxygen → calcium oxide (CaO); iron + sulfur → iron sulfide (FeS).
How do you name compounds containing polyatomic ions?
A polyatomic ion is a charged group of atoms that stays together in reactions. The most common ones have endings of -ate (containing oxygen) or -ite (same elements but fewer oxygen atoms):
| Polyatomic ion | Name | Example compound | Name |
|---|---|---|---|
| NO₃⁻ | Nitrate | KNO₃ | Potassium nitrate |
| SO₄²⁻ | Sulfate | CuSO₄ | Copper sulfate |
| CO₃²⁻ | Carbonate | CaCO₃ | Calcium carbonate |
| OH⁻ | Hydroxide | NaOH | Sodium hydroxide |
| PO₄³⁻ | Phosphate | Na₃PO₄ | Sodium phosphate |
| NH₄⁺ | Ammonium | NH₄Cl | Ammonium chloride |
The names of these ionic groups must be memorised; they do not follow a simple pattern from the element names. A useful cue: -ate compounds nearly always contain oxygen; -ite compounds contain the same elements as the corresponding -ate but with one fewer oxygen (e.g. sulfate SO₄²⁻ vs sulfite SO₃²⁻).
How do you name covalent molecular compounds?
Covalent compounds form between non-metals and use Greek numerical prefixes to show how many atoms of each element are in one molecule:
| Prefix | Number |
|---|---|
| mono- | 1 |
| di- | 2 |
| tri- | 3 |
| tetra- | 4 |
| penta- | 5 |
Rules:
- Name the first element with its prefix, then the second element with -ide ending and its prefix.
- Omit "mono-" before the first element only (it is assumed to be 1 if no prefix is given).
Examples:
| Formula | Name |
|---|---|
| CO | Carbon monoxide |
| CO₂ | Carbon dioxide |
| SO₂ | Sulfur dioxide |
| SO₃ | Sulfur trioxide |
| NO₂ | Nitrogen dioxide |
| N₂O₄ | Dinitrogen tetroxide |
| PCl₃ | Phosphorus trichloride |
Note that water (H₂O) and ammonia (NH₃) use their common names in everyday chemistry rather than "dihydrogen oxide" and "nitrogen trihydride", though the systematic names are technically correct.
How do you name acids?
Acids are named differently from other compounds:
- Binary acids (hydrogen + a non-metal): name as "hydro-[element]-ic acid". HCl = hydrochloric acid; HBr = hydrobromic acid; HI = hydroiodic acid.
- Oxoacids (contain oxygen): the name follows from the anion. If the anion ends in -ate, the acid ends in -ic acid; if the anion ends in -ite, the acid ends in -ous acid.
| Anion | Anion name | Acid | Acid name |
|---|---|---|---|
| NO₃⁻ | Nitrate | HNO₃ | Nitric acid |
| SO₄²⁻ | Sulfate | H₂SO₄ | Sulfuric acid |
| CO₃²⁻ | Carbonate | H₂CO₃ | Carbonic acid |
| SO₃²⁻ | Sulfite | H₂SO₃ | Sulfurous acid |
What are the most common naming mistakes to avoid?
- Swapping the order: it is always metal first, then non-metal. "Chloride sodium" is never correct — the name is "sodium chloride".
- Using -ide when there is oxygen: if the compound contains oxygen (other than for an oxide itself), the anion ending is usually -ate or -ite, not -ide. "Sodium sulfide" (Na₂S) contains only sulfur, not oxygen; "sodium sulfate" (Na₂SO₄) contains oxygen.
- Forgetting prefixes in covalent compounds: CO and CO₂ are very different substances (toxic gas vs greenhouse gas). The prefix di- is critical.
- Changing the metal's name: the metal name is always unchanged. Iron stays "iron", not "ironide". Only the non-metal component changes to its -ide form.
- Using prefixes in ionic compounds: Greek prefixes (mono-, di-, tri-) are for covalent molecular compounds only. You do not write "disodium oxide" — that is a covalent naming convention applied wrongly to an ionic compound.
Frequently asked questions
What does the -ide ending mean in chemistry?
The -ide ending indicates that the compound contains only two elements (binary compound) with no oxygen in the anion, or that the anion is a single non-metal ion with a negative charge. For example, "chloride" (Cl⁻), "sulfide" (S²⁻), "nitride" (N³⁻), and "oxide" (O²⁻) all use -ide. If oxygen is present and the anion contains more than one element, the ending changes to -ate (with more oxygen) or -ite (with less oxygen).
What is the difference between sulfate and sulfide?
Sulfate (SO₄²⁻) is a polyatomic ion containing one sulfur atom bonded to four oxygen atoms. Sulfide (S²⁻) is a single sulfur ion with no oxygen. Sodium sulfate (Na₂SO₄) and sodium sulfide (Na₂S) are completely different compounds with different properties. The -ate suffix always signals the presence of oxygen in the polyatomic anion; the -ide suffix signals a simple anion with no oxygen.
Why do we use Greek prefixes for some compounds but not others?
Greek prefixes (mono-, di-, tri-, etc.) are used for covalent molecular compounds between non-metals, where the number of atoms in a molecule is fixed and chemically significant (CO and CO₂ are entirely different substances with different properties). Ionic compounds do not need prefixes because the ratio of ions is determined by the charges: sodium (Na⁺) combined with oxide (O²⁻) can only form Na₂O — the 2:1 ratio is implied by the charges and does not need a prefix.
How do you name a compound if you are given its formula?
Work through this checklist: (1) Does it contain a metal? If yes, write the metal name first. (2) Is the anion a single element? Name it with -ide. Is it a polyatomic group? Use the group's name (-ate or -ite). (3) Is it a compound between two non-metals? Use Greek prefixes to count the atoms. (4) Does it start with H? If yes, it may be an acid — use the acid naming rules. Practising with a variety of formulae is the fastest way to become confident with the patterns.
For Socratic KS3 chemistry with Professor Curie — predicting a compound's name from the particle-level picture of which ions it contains before looking at any naming table — visit aitutors.me.