Introduction
When two non-metals bond together, they form molecular (covalent) compounds rather than ionic ones — think of things like carbon dioxide, sulfur hexafluoride, or nitrogen dioxide. Because non-metals share electrons rather than transfer them, we can't name these compounds the same way we name ionic compounds (like sodium chloride). Instead, chemists use a system of prefixes to show exactly how many atoms of each element are present. This naming system was standardised by IUPAC, whose job is to make chemical names unambiguous and consistent across the world (Talbot, p. 172).
Why We Need Prefixes
Non-metals can combine in several different ratios. For example, nitrogen and oxygen can form NO, NO₂, N₂O, N₂O₃, N₂O₄, and N₂O₅ — all different compounds with very different properties! If we just called them all "nitrogen oxide," it would be impossible to know which one we meant. Prefixes solve this problem by stating the exact number of atoms of each element in the formula.
The Prefix System
| Number of atoms | Prefix |
|---|---|
| 1 | mono- |
| 2 | di- |
| 3 | tri- |
| 4 | tetra- |
| 5 | penta- |
| 6 | hexa- |
| 7 | hepta- |
Key rules:
- The less electronegative element (usually further left/down the periodic table) is named first, and the more electronegative element is named second, ending in -ide.
- The prefix mono- is usually omitted for the first element (we don't say "monocarbon dioxide," just "carbon dioxide") but it is used for the second element when there's only one atom of it (e.g. carbon monoxide).
- When a prefix ending in a vowel (like mono- or penta-) meets an element name starting with a vow