Naming Compounds

One set of rules turns any formula into a name — and any name back into a formula.

High schoolIntro Gen ChemUni Year 1
⏱️ About 18 min

Chemistry has millions of compounds, and nobody memorises millions of names. Instead there's a small rulebook: learn how the pieces fit and you can name a compound you've never seen — and write its formula from the name alone. It's less vocabulary than it looks.

💡
The big idea: Naming follows the bonding. Metal + non-metal makes an ionic compound named by its ions; two non-metals make a covalent compound named with Greek prefixes that count the atoms. Get the compound type right and the name falls out of a short set of rules.
🎯 By the end, you'll be able to
  • Tell ionic from covalent (molecular) compounds and name each correctly
  • Use common polyatomic ions and balance charges to write neutral formulas
  • Name simple acids from their anions
📎 Helpful to know first

First decide: ionic or covalent?

Every naming decision starts with one question: what kind of compound is this? The answer sets which rulebook you use.

  • Ionic — a metal joined to a non-metal (e.g. NaCl, CaF₂). Held together by the attraction between positive and negative ions.
  • Covalent (molecular) — two non-metals (e.g. CO₂, N₂O₄). Atoms share electrons.

Metals sit on the left of the periodic table, non-metals on the right. That single split decides everything that follows.

🔑 Naming ionic compounds
Name the cation (positive, usually the metal) first, then the anion (negative) with its ending changed to -ide. So Na⁺ + Cl⁻ → sodium chloride; Mg²⁺ + O²⁻ → magnesium oxide. No prefixes — the charges already fix the ratio.

Charges fix the formula

An ionic compound is overall neutral, so the positive and negative charges must cancel. Magnesium is Mg²⁺ and chlorine is Cl⁻; you need two Cl⁻ to balance one Mg²⁺, giving MgCl₂. A quick shortcut is the ‘criss-cross’: each ion's charge number becomes the other ion's subscript (then reduce to the simplest ratio).

Some metals form more than one charge. Iron can be Fe²⁺ or Fe³⁺, so we put the charge in Roman numerals: iron(II) chloride is FeCl₂ and iron(III) chloride is FeCl₃.

✨ Polyatomic ions travel as a unit
Some ions are whole groups of atoms with an overall charge — polyatomic ions. The common ones to know: nitrate NO₃⁻, sulfate SO₄²⁻, carbonate CO₃²⁻, phosphate PO₄³⁻, hydroxide OH⁻, and ammonium NH₄⁺ (the one common positive one). Treat each as a single unit: if you need two of them, wrap it in brackets — calcium nitrate is Ca(NO₃)₂.
\[ \text{Al}^{3+} + \text{SO}_4^{2-} \;\longrightarrow\; \text{Al}_2(\text{SO}_4)_3 \]
Charges 3+ and 2- balance at a 2:3 ratio, so aluminium sulfate is Al₂(SO₄)₃. The bracket keeps the sulfate unit intact.
🔑 Naming covalent compounds
For two non-metals, use Greek prefixes to count each atom: mono(1), di(2), tri(3), tetra(4), penta(5). CO is carbon monoxide, CO₂ is carbon dioxide, N₂O₄ is dinitrogen tetroxide. (We drop ‘mono’ on the first element: it's carbon dioxide, not monocarbon dioxide.)
✨ A quick word on acids
Acids in water are named from their anion. An -ide anion gives a hydro…-ic acid: HCl is hydrochloric acid. An -ate anion gives an …-ic acid: H₂SO₄ (sulfate) is sulfuric acid, HNO₃ (nitrate) is nitric acid. An -ite anion gives an …-ous acid.
📝 Worked example: Write the formula for calcium phosphate.
  1. Identify the ions: calcium is Ca²⁺; phosphate is the polyatomic ion PO₄³⁻.
  2. Balance the charges: two Ca²⁺ give +4, three PO₄³⁻ give −6 — not balanced. Find the lowest common multiple of 2 and 3, which is 6: use three Ca²⁺ (+6) and two PO₄³⁻ (−6).
  3. Write it neutral, bracketing the polyatomic unit: Ca₃(PO₄)₂.
✓ Ca₃(PO₄)₂ — three calcium ions to two phosphate ions.
✏️ Practice: Aluminium sulfate is Al₂(SO₄)₃. How many oxygen atoms are in one formula unit? (Each sulfate, SO₄, contains 4 oxygen atoms.)
O atoms
Solution
  1. The subscript 3 outside the bracket multiplies everything inside it.
  2. Each SO₄ has 4 oxygen atoms, and there are 3 sulfate units: 4 × 3.
  3. = 12 oxygen atoms. (The formula also has 2 Al and 3 S.)

Check your understanding

1. Which compound is named with Greek prefixes like 'di' and 'tetra'?
Prefixes are for covalent compounds — two non-metals, like N₂O₄ (dinitrogen tetroxide). Ionic compounds (a metal + a non-metal) use charges, not prefixes.
2. What is the formula for magnesium chloride, given Mg²⁺ and Cl⁻?
One Mg²⁺ needs two Cl⁻ to cancel the +2 charge, giving MgCl₂ — a neutral compound.
3. Iron(III) oxide contains which iron ion?
The Roman numeral (III) states the charge directly: iron(III) is Fe³⁺. That's why iron(III) oxide is Fe₂O₃.
🔎 Related lessons
Next: The Mole & Molar Mass Balancing Chemical Equations
✅ Key takeaways
  • Decide the compound type first: metal + non-metal = ionic; two non-metals = covalent.
  • Ionic: name cation then anion (-ide ending); balance charges to get the formula.
  • Polyatomic ions (NO₃⁻, SO₄²⁻, CO₃²⁻, PO₄³⁻, OH⁻, NH₄⁺) move as one unit — bracket them.
  • Covalent: use prefixes (mono, di, tri…) to count each atom.
  • Acids are named from their anion: -ide → hydro…-ic; -ate → …-ic; -ite → …-ous.
➡️ Now you can read and write formulas fluently. Next we make formulas quantitative: the mole lets you count atoms by weighing them, which is the gateway to every calculation in this module.
Want to test yourself on this? Try the Chemistry practice test →