How to Name Covalent Compounds Using Prefixes: A Practical Guide
To name a covalent compound, name the first element, then name the second element with its ending changed to -ide. Add a Greek prefix to show the number of atoms of each element. The main exception is that mono- is omitted from the first element.
For example, N₂O₅ is dinitrogen pentoxide: di- indicates two nitrogen atoms, and pent- indicates five oxygen atoms.
Check that the compound is covalent
The prefix system is used for many compounds made from two nonmetals. These elements share electrons. Compounds made from a metal and a nonmetal are usually named as ionic compounds, which follow different rules. Polyatomic ions, such as nitrate and sulfate, also use ionic naming conventions. OpenStax explains the distinction between these naming systems in its section on chemical nomenclature.
For a binary covalent compound, identify the two elements and count their atoms from the formula. When deciding which element to name first, a common introductory rule is to put the element farther left on the periodic table first. If both elements are in the same group, the one lower in the group goes first. Use the naming convention specified by your course or reference when it provides a different ordering rule.
Choose prefixes from the atom counts
Use the subscripts in the formula to select prefixes. If an element has no written subscript, its count is one.
| Number of atoms | Prefix |
|---|---|
| 1 | mono- |
| 2 | di- |
| 3 | tri- |
| 4 | tetra- |
| 5 | penta- |
| 6 | hexa- |
| 7 | hepta- |
| 8 | octa- |
| 9 | nona- |
| 10 | deca- |
Apply the prefix to each element name according to its atom count. The first element keeps its element name. The second element takes the -ide ending: oxygen becomes oxide, chlorine becomes chloride, and sulfur becomes sulfide.
There is one important exception to applying the table mechanically: omit mono- when the first element has one atom. Keep mono- for the second element when its count is one. This distinction helps show the full composition of the compound.
Combine the parts of the name
Write the first element’s prefix and name, followed by the second element’s prefix and modified name. Leave a space between the two element names. Do not add spaces between a prefix and the element name it modifies.
Prefixes that end in a vowel sometimes lose that vowel before an element name that begins with a vowel. This avoids awkward consecutive vowel sounds. For instance, mono- and oxide combine as monoxide, while tetra- and oxide combine as tetroxide. The count is still conveyed by the prefix: the spelling change does not alter the number of atoms.
One worked example: N₂O₅
- Identify the elements. The formula contains nitrogen and oxygen, both nonmetals.
- Name the first element. Nitrogen comes before oxygen under the common periodic-table ordering rule. The formula has two nitrogen atoms, so use di- to form dinitrogen.
- Name the second element. There are five oxygen atoms, so use penta-. Change oxygen to oxide. The final a in penta- is dropped before oxide, giving pentoxide.
- Put the name together. The compound is dinitrogen pentoxide.
The formula and name should agree in both atom counts: di- represents two nitrogen atoms, and penta- represents five oxygen atoms. This check can catch errors such as reversing the counts or overlooking a subscript.
Key Takeaways
- Use prefixes to name many binary covalent compounds made from two nonmetals.
- Name the first element, then the second element with an -ide ending.
- Choose each prefix from the number of atoms shown in the formula.
- Omit mono- on the first element, but retain it on the second when needed.
- Drop a prefix’s final vowel before a vowel at the start of the next element name when standard spelling calls for it, as in pentoxide.
