Why learn two sets of names?
The AP exam and English-language textbooks use English names only, while Chinese textbooks and exams use Chinese names. is “iron(II) chloride” in English and 氯化亚铁 in Chinese. Both systems follow clear rules, and comparing them side by side actually makes both easier to remember.
English names
1. Ionic compounds (a metal with a nonmetal, or containing a polyatomic ion): cation name + anion name.
- An anion made of a single element ends in -ide: chlorine → chloride, oxygen → oxide, sulfur → sulfide. is sodium chloride and is calcium oxide.
- A polyatomic ion keeps its own name: sodium sulfate, calcium carbonate, sodium hydroxide, ammonium chloride.
- Ionic compounds don’t use number prefixes: is calcium chloride, not calcium dichloride.
2. Metals with more than one valence: add a Roman numeral in parentheses after the metal.
- iron(III) chloride, iron(II) chloride, copper(II) oxide, copper(I) oxide
3. Molecular compounds of two nonmetals: use prefixes for the number of atoms.
| Number | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Prefix | mono- | di- | tri- | tetra- | penta- |
- carbon monoxide, carbon dioxide, diphosphorus pentoxide, dinitrogen tetroxide
- Leave out mono- for the first element: is carbon dioxide, not monocarbon dioxide.
Chinese names
If you also study in Chinese, the rules map neatly onto the English ones:
- Two elements: “某化某,” read from back to front. is 氯化钠 (literally “chloride-sodium”). For molecular compounds, the numbers are read out: is 五氧化二磷 (“five-oxide two-phosphorus”).
- Polyatomic ions: sulfate, carbonate, and nitrate compounds are “某酸某”: 硫酸钠. Hydroxides are “氢氧化某”: 氢氧化钠.
- The lower of two valences gets the character 亚 instead of a Roman numeral: 氯化铁, 氯化亚铁.
| Formula | English | Chinese |
|---|---|---|
| sodium chloride (front to back) | 氯化钠 (back to front) | |
| sodium sulfate | 硫酸钠 | |
| iron(II) chloride (Roman numeral) | 氯化亚铁 (亚) | |
| diphosphorus pentoxide (prefixes) | 五氧化二磷 (numbers read out) |
The easiest thing to mix up: English reads front to back, Chinese reads back to front.
Example 1: Naming
- Look at what it’s made of. Iron (a metal) and chlorine, so it’s an ionic compound.
- Find iron’s valence. Cl is −1, so x + (−1) × 2 = 0 and x = +2.
- English name. Iron has more than one valence, so add a Roman numeral: iron(II) chloride.
- Chinese name. +2 is iron’s lower valence, so it gets 亚: 氯化亚铁.
Example 2: Naming
- Look at what it’s made of. N and O are both nonmetals, so it’s a molecular compound.
- English name. Front to back, with prefixes: dinitrogen pentoxide (drop the a when penta- meets oxide).
- Chinese name. Back to front, with the numbers read out: 五氧化二氮.
Common mistakes
Mistake 1: Reversing the order. is sodium chloride, not chloride sodium. (In Chinese it’s the other way round: 氯化钠.)
Mistake 2: Leaving out the Roman numeral. is iron(II) chloride. Just “iron chloride” doesn’t tell you which of the two iron chlorides you mean.
Mistake 3: Using prefixes for ionic compounds. is calcium chloride, not calcium dichloride. Prefixes are only for compounds of two nonmetals.
Quick check
What is the English name of ?
O is −2, so 2x + (−2) × 3 = 0 and iron is +3. Iron has more than one valence, so the name needs (III). It's ionic, so no prefixes.
What is the English name of ?
Cl is −1, so copper is +2. Copper can be +1 or +2, so the name needs (II), and ionic compounds don't use prefixes like di-.
Summary
- Ionic compounds: cation name + anion name, with -ide for single-element anions and no prefixes.
- Metals with more than one valence get a Roman numeral, such as iron(II).
- Compounds of two nonmetals use prefixes: mono-, di-, tri-, tetra-, penta-.
- Chinese names read back to front and use 亚 for the lower valence.
On to AP (for later)
AP questions often give a substance’s name only, so you need to write its formula yourself. The full set of English naming rules, including more polyatomic ions and acids, comes in Stage 2 (2.5 Nomenclature).