How to Read Chemical Formulas
By the Periodixy Editorial Team · Last reviewed July 10, 2026
A chemical formula is a compressed sentence. H₂O says: “this molecule contains 2 hydrogen atoms bonded with 1 oxygen atom.” Once you can decode symbols, subscripts, parentheses and coefficients, no formula — however long — is confusing.
The four building blocks
- Element symbols: one capital letter, optionally one lowercase (H, He, Na). Capitalisation is meaningful: CO is carbon monoxide; Co is cobalt.
- Subscripts: the small number after a symbol counts its atoms. No subscript means 1. H₂O = 2 H + 1 O.
- Parentheses: group atoms that act as a unit; the subscript after the parenthesis multiplies everything inside. Ca(OH)₂ = 1 Ca + 2 O + 2 H.
- Coefficients: full-size numbers in front multiply the entire formula. 3H₂O means three water molecules = 6 H + 3 O.
Worked decodings
Aluminium sulfate, Al₂(SO₄)₃
Count every atom in Al₂(SO₄)₃.
- Al: subscript 2 → 2 aluminium.
- The (SO₄) group is taken 3 times.
- S: 1 × 3 = 3 sulfur. O: 4 × 3 = 12 oxygen.
Answer: 2 Al, 3 S, 12 O — 17 atoms total.
A hydrate: CuSO₄·5H₂O
What does the dot in CuSO₄·5H₂O mean?
- The dot attaches 5 water molecules to each CuSO₄ unit in the crystal.
- Water part: 5 × (2 H + 1 O) = 10 H + 5 O.
- Total: 1 Cu, 1 S, 4 + 5 = 9 O, 10 H.
Answer: 1 Cu, 1 S, 9 O, 10 H. Hydrates are common in blue copper sulfate crystals.
Polyatomic ions: recognise the squads
Certain atom groups travel together so often they have their own names. Spotting them makes formulas readable at a glance:
| Ion | Formula | Found in |
|---|---|---|
| Hydroxide | OH⁻ | NaOH, Ca(OH)₂ |
| Sulfate | SO₄²⁻ | H₂SO₄, CuSO₄ |
| Nitrate | NO₃⁻ | KNO₃, AgNO₃ |
| Carbonate | CO₃²⁻ | CaCO₃, Na₂CO₃ |
| Phosphate | PO₄³⁻ | Ca₃(PO₄)₂ |
| Ammonium | NH₄⁺ | NH₄Cl, (NH₄)₂SO₄ |
Practice by pasting any formula into the Molar Mass Calculator — its breakdown table shows exactly how the formula was counted.
Summary
- Symbols name elements; capitalisation matters (CO ≠ Co).
- Subscripts count atoms; parentheses group units; coefficients multiply whole formulas.
- Hydrate dots attach water molecules: CuSO₄·5H₂O.
- Learn the common polyatomic ions to read formulas in chunks, not letters.