Naming with prefixes
Naming covalent compounds.
Why is CO₂ called carbon dioxide?
The formula CO₂ shows one carbon atom and two oxygen atoms. Its name tells us the same thing.
Which part of the name do you think tells us there are two oxygen atoms?
What this lesson covers
The idea
Covalent compounds are named with prefixes (mono-, di-, tri-, tetra-, penta-, hexa-) for numbers of atoms; the first element keeps its name without mono-, the second ends in -ide, and a prefix's final 'o' or 'a' drops before a vowel.
Why is CO₂ called carbon dioxide?
The formula CO₂ shows one carbon atom and two oxygen atoms. Its name tells us the same thing.
Which part of the name do you think tells us there are two oxygen atoms?
- The prefix di- in dioxide
- The word carbon
- The ending -ide
Read the names
Pick a compound. Count the atoms and read the prefixes in the name.
The prefix system
A prefix counts the atoms of each element. The first element keeps its regular name and the second element ends in -ide. Mono- is usually left out for the first element but used for the second. A final 'o' or 'a' of a prefix is dropped before a vowel, as in monoxide, tetroxide and pentoxide, but 'i' is kept, as in dioxide.
Covalent compounds are named with prefixes (mono- 1, di- 2, tri- 3, tetra- 4, penta- 5, hexa- 6) for the number of atoms. The first element keeps its name without mono-, the second ends in -ide, and a prefix's final 'o' or 'a' drops before a vowel.
Notes
Prefixes (mono-, di-, tri-, tetra-, penta-, hexa-) give the number of atoms. The first element keeps its name (no mono-), the second ends in -ide.
Check yourself
What is CO named?
What is the correct name of N₂O₄?
PCl₃ is named phosphorus trichloride. How many chlorine atoms does one molecule have?
Answer: 3
The prefix tri- means three, so there are 3 chlorine atoms.
Which formula is sulfur hexafluoride?
- Carbon monooxide. The 'o' of mono- is dropped before a vowel, so it is monoxide.
- Carbon monoxide — correct. Yes!
- Monocarbon monoxide. Mono- is usually omitted for the first element, so the name starts with carbon.
- Dinitrogen tetraoxide. The 'a' of tetra- is dropped before a vowel, so it is tetroxide.
- Nitrogen tetroxide. There are 2 nitrogen atoms, and the prefix di- shows that.
- Dinitrogen tetroxide — correct. Yes!
- SF₆ — correct. Yes!
- SF₄. The prefix hexa- means six, so there are six fluorine atoms, not four.
- S₆F. The prefix hexa- is on fluoride, so it is six fluorine atoms and one sulfur atom.