Chemistry 20 · Names and formulas
The naming system, learned once
This is the least interesting unit in the course and the most expensive to get wrong, because every later question is written in this language. An hour spent getting it automatic pays back all year.
- 1. The words, first
- 2. Naming ionic compounds
- 3. Naming molecular compounds
- 4. Naming simple organic compounds
- 5. What costs marks
The words, first
The idea: Three naming systems, and the first job in any question is working out which one applies.
| Word | What it means |
|---|---|
| Formula unit | The simplest whole-number ratio of ions in an ionic compound. NaCl, not Na₂Cl₂. |
| Molecule | A discrete group of covalently bonded atoms. Molecular formulas are not reduced: H₂O₂ is not HO. |
| Monatomic ion | An ion made from one atom: Na⁺, Cl⁻, O²⁻. |
| Polyatomic ion | A charged group of atoms that stays together: NO₃⁻, SO₄²⁻, NH₄⁺, CO₃²⁻, PO₄³⁻. |
| Multivalent metal | A metal that forms more than one ion, so the name needs a Roman numeral: iron(II) and iron(III). |
| Roman numeral | States the charge on the metal, not the number of anions. FeCl₃ is iron(III) chloride because one Fe³⁺ needs three Cl⁻. |
| Prefix | mono-, di-, tri-, tetr-, pent-, hex-. Used for molecular compounds only, and mono- is dropped on the first element. |
| Hydrocarbon | A compound of hydrogen and carbon only. Alkanes end in -ane, alkenes in -ene, alkynes in -yne. |
Naming ionic compounds
The idea: Metal first, non-metal second with an -ide ending, and subscripts that come from balancing charge rather than from the name.
From name to formula. Write the two ions with their charges, then take the lowest whole-number ratio that balances. Aluminum oxide: Al³⁺ and O²⁻ need six units of charge each way, so Al₂O₃.
From formula to name. Name the metal, then the non-metal with -ide: MgO is magnesium oxide. No prefixes — the charges already fix the ratio.
Multivalent metals need a numeral. For Cu(NO₃)₂, two nitrates carry 2−, so copper must be Cu²⁺: copper(II) nitrate. Work the charge out from the anions rather than guessing.
Polyatomic ions get brackets when more than one is needed, so the subscript applies to the whole group: (NH₄)₂SO₄, Fe₃(PO₄)₂.
-ide, -ate, -ite. An -ide ending is usually a single atom (chloride, Cl⁻); -ate and -ite are polyatomic ions containing oxygen, with -ite having one fewer oxygen than -ate (nitrate NO₃⁻, nitrite NO₂⁻). Confusing nitride with nitrate is a standard trap.
Naming molecular compounds
The idea: Two non-metals can combine in several ratios, so the name has to say which one — that is what the prefixes are for.
Both elements take prefixes, except that mono- is dropped on the first: CO is carbon monoxide, CO₂ is carbon dioxide, N₂O₄ is dinitrogen tetroxide.
Vowels get dropped where the prefix would collide: tetroxide rather than tetraoxide, pentoxide rather than pentaoxide.
Deciding which system to use. Metal + non-metal → ionic, no prefixes. Non-metal + non-metal → molecular, prefixes. Checking the periodic table takes two seconds and prevents a whole class of error.
Naming simple organic compounds
The idea: Count the carbons for the prefix, look at the bonds for the ending.
1 meth- 2 eth- 3 prop- 4 but- 5 pent- 6 hex- 7 hept- 8 oct-
-ane all single bonds · -ene a double bond · -yne a triple bond
Worked example. CH₃CH₂CH₂CH₃ has four carbons and only single bonds: butane. CH₂=CHCH₃ has three carbons and a double bond: propene.
Chemistry 20 asks only for the simple straight-chain cases. Branching, numbering and functional groups are Chemistry 30's problem.
What costs marks
The idea: These are all avoidable in a way that the calculation units are not.
- Using prefixes on an ionic compound. CaCl₂ is calcium chloride, never calcium dichloride.
- Reading the Roman numeral as a count. Iron(III) chloride has three chlorides because iron is 3+, but the numeral names the charge.
- Forgetting brackets around a repeated polyatomic ion. NH₄₂SO₄ means something different and usually nothing at all.
- Reducing a molecular formula. H₂O₂ is hydrogen peroxide; HO is not a thing.