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Carbon monoxide and carbon dioxide differ by a single oxygen atom. One is a colourless, odourless poison; the other is what you exhale with every breath. When two nonmetals can combine in several different ratios, the name has to tell you exactly which compound you mean. That’s what the prefix system for covalent compounds does.
Ionic vs. covalent naming
For ionic compounds, the charges of the ions fix the ratio, so names don’t need numbers: CaCl₂ is just calcium chloride.
Covalent (molecular) compounds are formed between nonmetals, with no ion charges to fix the ratio. Nitrogen and oxygen alone form NO, NO₂, N₂O, N₂O₄ and N₂O₅. So covalent names spell out the number of each atom with Greek prefixes.
The prefixes
| Number | Prefix |
|---|---|
| 1 | mono- |
| 2 | di- |
| 3 | tri- |
| 4 | tetra- |
| 5 | penta- |
| 6 | hexa- |
| 7 | hepta- |
| 8 | octa- |
| 9 | nona- |
| 10 | deca- |
The rules
- Name the first element (with a prefix if there’s more than one of it).
- Name the second element with a prefix and the ending -ide.
- Never use “mono-” on the first element. CO is carbon monoxide, not monocarbon monoxide.
- Drop the final “a” or “o” of the prefix before a vowel, especially before “oxide”: monoxide not monooxide, pentoxide not pentaoxide, tetroxide not tetraoxide. (Some sources keep “tetraoxide”; either is understood.)
Which element goes first?
The element further to the left in the periodic table usually goes first — the less electronegative one. If both are in the same group, the lower one goes first. In practice, formulas follow this order:
B, Si, C, Sb, As, P, N, H, Te, Se, S, At, I, Br, Cl, O, F
That’s why it’s “sulfur dioxide” (SO₂) but “oxygen difluoride” (OF₂): fluorine comes after oxygen in the list, so oxygen is named first.
Worked examples
| Formula | Name |
|---|---|
| CO | carbon monoxide |
| CO₂ | carbon dioxide |
| NO₂ | nitrogen dioxide |
| N₂O | dinitrogen monoxide (laughing gas) |
| N₂O₄ | dinitrogen tetroxide |
| N₂O₅ | dinitrogen pentoxide |
| SO₃ | sulfur trioxide |
| PCl₃ | phosphorus trichloride |
| PCl₅ | phosphorus pentachloride |
| SF₆ | sulfur hexafluoride |
| CCl₄ | carbon tetrachloride |
| P₄O₁₀ | tetraphosphorus decoxide |
| Cl₂O₇ | dichlorine heptoxide |
| XeF₄ | xenon tetrafluoride |
From name to formula
Just translate the prefixes back into subscripts. No charge balancing needed.
- Dinitrogen trioxide → N₂O₃
- Silicon tetrachloride → SiCl₄
- Iodine heptafluoride → IF₇
The exceptions everyone uses
Some molecules are almost never called by their systematic names:
| Formula | Common name | Systematic name |
|---|---|---|
| H₂O | water | dihydrogen monoxide |
| NH₃ | ammonia | azane |
| CH₄ | methane | (already systematic) |
| H₂O₂ | hydrogen peroxide | dihydrogen dioxide |
| N₂H₄ | hydrazine | diazane |
“Dihydrogen monoxide” is famous mostly as a joke — petitions to ban this “dangerous chemical” have fooled people for years. It’s water.
Hydrocarbons (compounds of carbon and hydrogen) have their own naming system — methane, ethane, propane and so on — which is part of organic chemistry rather than the prefix system.
Binary acids
Compounds of hydrogen with a halogen are named as covalent compounds when pure, and as acids when dissolved in water:
- HCl(g) is hydrogen chloride; HCl(aq) is hydrochloric acid.
- HBr(g) is hydrogen bromide; HBr(aq) is hydrobromic acid.
Quick answers
How do I know whether to use prefixes? If the compound is made only of nonmetals (and isn’t an ammonium compound), use prefixes. If it contains a metal, it’s usually ionic — no prefixes.
Why is it “carbon monoxide” but not “monocarbon”? By convention, mono- is dropped from the first element. A lone first element is assumed to be one.
Is it “tetroxide” or “tetraoxide”? Both appear. IUPAC’s recommendations keep the full prefix (“tetraoxide”), but “tetroxide” is very widely used and is what most textbooks teach.
Related
For bonding background, see ionic vs. covalent bonds. The molar mass calculator will give the molar mass of any formula once you’ve written it.
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