Naming: a decision tree
Chemical names are not arbitrary. They are a code with about six rules, and every rule is there to remove an ambiguity. Learn it as a decision tree and it becomes mechanical.
Why is there a naming system at all?
Before the 1780s, substances were named after where they came from, who made them or what they looked like — oil of vitriol, butter of antimony, Glauber's salt. Two chemists in different cities could not be sure they were discussing the same compound, and a written recipe was close to useless outside the room it was written in.
The modern system fixes that by making the name describe the composition. Given the name, anybody can write the formula; given the formula, anybody can write the name. That is the entire point, and it is why the rules are so fussy about prefixes and Roman numerals — each one removes a case where two different compounds would otherwise share a name.
In practice this matters for you because every later question is written in this language. A stoichiometry question you could do perfectly is unanswerable if you cannot turn iron(III) chloride into FeCl₃.
Where it turns up
- Reading a label — the ingredients on food, cleaning products and medicines use these names
- Safety — sodium nitrate and sodium nitrite differ by one letter and one oxygen, and behave very differently
- Any lab instruction — a procedure names its reagents and assumes you can find them
- Chemistry 30 — organic chemistry is an extension of exactly this system
The words, first
The idea: The terms that decide which branch of the tree you are on.
| Word | What it means |
|---|---|
| Ionic compound | Metal plus non-metal. Electrons transfer, ions form, no prefixes. |
| Molecular compound | Non-metal plus non-metal. Electrons are shared, and prefixes are needed. |
| Formula unit | The simplest whole-number ratio of ions. NaCl, never Na₂Cl₂. |
| Multivalent metal | Forms more than one ion, so the name needs a Roman numeral. Iron, copper, lead, tin, chromium, manganese. |
| Roman numeral | States the charge on the metal, worked out from the anions. |
| Polyatomic ion | A charged group that stays together: NO₃⁻, SO₄²⁻, CO₃²⁻, PO₄³⁻, NH₄⁺, OH⁻. |
| -ide, -ate, -ite | -ide is usually a single atom; -ate and -ite are oxygen-containing polyatomic ions, with -ite having one fewer oxygen. |
The decision tree
The idea: Four questions, asked in order. Each answer removes half the possibilities.
- Is the first element a metal? No → molecular, use prefixes, stop here. Yes → ionic, continue.
- Is that metal multivalent? Yes → work out its charge from the anions and write a Roman numeral. No → no numeral.
- Is the anion polyatomic? Yes → use its own name unchanged (nitrate, sulfate, carbonate). No → element name with -ide.
- Balance the charges to get the subscripts, and bracket any polyatomic ion needed more than once.
Worked: Cu(NO₃)₂ → ? Copper is a metal, so ionic. Copper is multivalent, so a numeral is needed: two nitrates carry 2− in total, so copper must be 2+. Nitrate is polyatomic and keeps its name. Copper(II) nitrate.
Worked: iron(II) phosphate → ? Fe²⁺ and PO₄³⁻. Lowest common multiple of charge is six, so three irons and two phosphates, and the phosphate needs brackets. Fe₃(PO₄)₂.
The five traps
The idea: Almost every lost mark in this unit is one of these.
- Prefixes on an ionic compound. CaCl₂ is calcium chloride, never calcium dichloride. The charges already fix the ratio, so a prefix would be redundant.
- Reading the Roman numeral as a count. Iron(III) chloride has three chlorides because iron is 3+, but the numeral names the charge, not the number.
- Missing brackets. (NH₄)₂SO₄ and NH₄₂SO₄ are not the same thing; the second is not anything.
- Reducing a molecular formula. H₂O₂ is hydrogen peroxide; HO does not exist.
- -ide for -ate. Nitride is N³⁻; nitrate is NO₃⁻. One letter, completely different compound.
How to make it automatic
The idea: This is one of the few parts of chemistry where rote practice genuinely is the fastest route.
Learn the polyatomic ions cold. Nine of them cover nearly everything at this level: nitrate NO₃⁻, nitrite NO₂⁻, sulfate SO₄²⁻, sulfite SO₃²⁻, carbonate CO₃²⁻, phosphate PO₄³⁻, hydroxide OH⁻, ammonium NH₄⁺, acetate CH₃COO⁻.
Learn the multivalent metals and their common charges: iron 2+/3+, copper 1+/2+, lead 2+/4+, tin 2+/4+.
Practise in both directions. Name to formula and formula to name are different skills, and exams ask for both.
Then stop. Ten minutes a day for a week will get this to automatic, and it stays there. It is the best return on time available in the whole course.