Binary molecular (covalent) compounds

Binary molecular (covalent) compounds are formed as the result of a reaction between two nonmetals. Although there are no ions in these compounds, they are named in a similar manner to binary ionic compounds. The nomenclature of binary covalent compounds follows these rules:

1. The first element in the formula is given first, using the element’s full name.
2. The second element is named as if it were an anion.
3. Prefixes are used to denote the numbers of atoms present. If the first element exists as a single atom, the prefix mono- is omitted. For example, CO is called carbon monoxide rather than monocarbon monoxide.

These examples show how the rules are applied for the covalent compounds formed by nitrogen and oxygen:

To avoid awkward pronunciations, the final o or a of the prefix is often dropped when the element name begins with a vowel. For example, N₂O₄ is referred to as dinitrogen tetroxide, not dinitrogen tetraoxide, and CO is called carbon monoxide, not carbon monooxide.