When two nonmetals bond, they share electrons and make a covalent (molecular) compound. There are no charges to balance, so the name has to say how many of each atom there are. That's what the prefixes do.
First: ionic or covalent?
- Metal + nonmetal (or ammonium) → ionic → no prefixes (calcium chloride).
- Nonmetal + nonmetal → covalent → prefixes (carbon dioxide).
The prefixes
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|
| mono- | di- | tri- | tetra- | penta- | hexa- | hepta- | octa- | nona- | deca- |
The rules
- Name the first element, with a prefix if there's more than one: dinitrogen.
- Name the second element ending in -ide, always with a prefix: tetroxide.
- Leave mono- off the first element: CO is carbon monoxide.
- Drop a doubled vowel: monoxide, not mono-oxide.
So N₂O₄ = dinitrogen tetroxide, and PCl₅ = phosphorus pentachloride.
Common names win
A few compounds are always called by their common names: water (H₂O), ammonia (NH₃) and methane (CH₄). “Dihydrogen monoxide” is technically correct, which is why it's a famous chemistry joke.