Naming covalent compounds
Two nonmetals can combine in more than one ratio: carbon and oxygen form both CO and CO2. The name must say how many of each atom there are, using prefixes.
| Number | Prefix |
|---|---|
| 1 | mono |
| 2 | di |
| 3 | tri |
| 4 | tetra |
| 5 | penta |
| 6 | hexa |
Rules:
- Name the first element, with a prefix if there is more than one atom. Never use mono on the first element.
- Name the second element with a prefix and the ending -ide.
- Drop the final a or o of a prefix before “oxide”: monoxide, not monooxide.
Worked example: name N2O4
Two nitrogen atoms: dinitrogen. Four oxygen atoms: tetra plus oxide becomes tetroxide.
dinitrogen tetroxide
Worked example: write the formula of sulfur hexafluoride
No prefix on sulfur means 1. Hexa means 6 fluorine atoms.
SF6
Common mistake
Using prefixes for ionic compounds, or leaving them off covalent ones. Check the first element: if it is a metal, use the ionic rules instead.