Chapter 5 · Molecules and Compounds
5.8Naming Molecular Compounds
10 min · two checks
Predict
What is the name of N₂O₄?
The idea
- Apply prefixes to binary molecular compounds.
- Know when the prefix mono- is dropped.
Binary molecular compounds are named with Greek prefixes: mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca. The second element takes the -ide ending. CO is carbon monoxide, not monocarbon monoxide: the prefix mono- is usually omitted on the first element and kept on the second when there is only one. N₂O is dinitrogen monoxide. Drop a vowel if the prefix crashes into oxide: monoxide, not monooxide; tetroxide is the usual spelling of tetraoxide.
Do not use this system for ionic compounds, and do not use roman numerals here. The prefixes are doing the counting that charges do in the ionic system. Some compounds have common names you should still recognize: water for H₂O, ammonia for NH₃, methane for CH₄. In formulas of molecular compounds the subscripts are the prefixes, written in the order of the name.
Keep these
- Prefixes count atoms in binary molecular compounds.
- Second element ends in -ide. Mono- is omitted on the first element.
- Water, ammonia, and methane keep common names.
Worked path
Name Cl₂O₇ and write the formula for dinitrogen pentoxide.
Observe
Both are binary compounds of nonmetals, so use prefixes.