Introduction
When two non-metal elements combine, they form a binary covalent compound — a compound made of only two elements, joined by covalent bonds rather than ionic ones. Because non-metals have relatively high electronegativity values, the electrons are shared rather than transferred (Talbot, p. 196). Compounds like CO₂, SO₃, and PCl₅ all fall into this category, and today's lesson is about naming them correctly using IUPAC rules. This matters because unambiguous naming lets chemists anywhere in the world understand exactly which substance is meant — that's the whole purpose of IUPAC's nomenclature system (Talbot, p. 172).
The naming rule for binary non-metal compounds
Unlike ionic compounds, where charges tell you the ratio of ions automatically, covalent compounds often need numerical prefixes because the same two elements can combine in several different ratios (think CO vs. CO₂). The rules are:
- Name the first element as it is, using its normal element name.
- Name the second element by changing its ending to "-ide" (just like in ionic compounds).
- Add a Greek numerical prefix to each element name to show how many atoms of that element are present — except you normally drop "mono-" for the first element (not the second).
Greek prefixes to memorise
1 = mono- 6 = hexa-
2 = di- 7 = hepta-
3 = tri- 8 = octa-
4 = tetra- 9 = nona-
5 = penta- 10 = deca-
- If a prefix would end in a vowel right before another vowel (like "mono-oxide" or "penta-oxide"), the extra vowel is dropped for smoother pronunciation: monoxide, pentoxide.
- The element written first in the formula is usually the less electronegative one (closer to the metals in the periodic table), and it is named first.
Worked Examples
Example 1: CO
- One carbon, one oxygen → first element gets no prefix (mono- dropped) → carbon monoxide
Example 2: CO₂
- One carbon, two oxygens → carbon dioxide
Example 3: N₂O₄
- Two nitrogens, four oxygens → dinitrogen tetroxide (tetra- loses its "a" before "oxide")
Example 4: PCl₅
- One phosphorus, five chlorines → phosphorus pentachloride
A quick contrast with ionic naming
It's worth remembering why this system exists at all. For binary ionic compounds (metal + non-metal, like KF), you don't need prefixes — the fixed charges on the ions already tell you the ratio, so "potassium fluoride" is enough information (Talbot, p. 196). But for binary covalent compounds (two non-metals), there's no fixed charge to guide you, so prefixes are essential to avoid ambiguity — otherwise "nitrogen oxide" could mean NO, N₂O, NO₂, N₂O₃, N₂O₄, or N₂O₅!
A few special gaseous elements (O₂, H₂, N₂, and the halogens) exist naturally as diatomic molecules with purely molecular covalent bonding (Talbot, p. 196) — but these are elements, not compounds, so they don't need this naming system at all.
Key takeaways
- Binary covalent compounds contain two non-metal elements joined by shared electrons.
- Name the first element normally, then the second element with an "-ide" ending.
- Use Greek prefixes (mono-, di-, tri-, tetra-, penta-...) to show how many atoms of each element are present; drop "mono-" only for the first element.
- Drop a vowel in the prefix when it clashes with the vowel starting the element name (e.g., pentoxide, not pentaoxide).
- This prefix system exists because covalent compounds, unlike ionic ones, don't have fixed charges to reveal their formula ratio automatically.