How to Write Chemical Formulas from Names

Being able to turn a name like “aluminium sulfate” or “dinitrogen tetroxide” into a correct formula is a core chemistry skill — you need it for balancing equations, calculating molar masses and understanding reactions. The good news is that you don’t have to memorise thousands of formulas. A handful of rules, plus a short list of common ions, lets you work out almost any formula from its name. This guide sets out the decision process and works through each type of compound.

Step 0: decide what type of compound it is

Look at the name and ask:

Clue in the name	Type	Method
Starts with a metal (or ammonium)	Ionic	Balance charges
Two non-metals, with prefixes like di-, tri-	Covalent (molecular)	Read the prefixes
Ends in “acid”	Acid	Anion + enough H⁺
Ends in “…hydrate”	Hydrate	Salt + dot + water

Part 1: ionic compounds

The rule

An ionic compound has no overall charge. Find the charge on each ion, then combine the smallest numbers of each so the charges cancel (see ionic bonding explained).

Step-by-step

1. Write the symbol and charge of the cation (the metal, named first).
2. Write the symbol and charge of the anion (named second).
3. Balance the charges with the smallest whole-number ratio.
4. Write the formula: cation first, subscripts for numbers, brackets around polyatomic ions if more than one is needed.

Finding ion charges

• Main-group metals: from the group — group 1: +1, group 2: +2, aluminium: +3.
• Transition metals: from the Roman numeral in the name — iron(III) = Fe³⁺ (see naming compounds with transition metals).
• Simple anions (ending -ide): from the group — group 17: −1 (chloride), group 16: −2 (oxide, sulfide), group 15: −3 (nitride).
• Polyatomic ions: learn the common ones (see common ions and their charges).

Worked examples

Sodium oxide: Na⁺ and O²⁻. Need two Na⁺ for one O²⁻ → Na₂O.

Calcium chloride: Ca²⁺ and Cl⁻. Need two Cl⁻ → CaCl₂.

Aluminium oxide: Al³⁺ and O²⁻. Lowest common multiple of 3 and 2 is 6 → 2 Al³⁺ (+6) and 3 O²⁻ (−6) → Al₂O₃.

Magnesium nitride: Mg²⁺ and N³⁻. LCM of 2 and 3 is 6 → 3 Mg²⁺ and 2 N³⁻ → Mg₃N₂.

The criss-cross method is a quick shortcut for this balancing step.

With polyatomic ions

Polyatomic ions stay together as a unit. If you need more than one, put the ion in brackets with the number outside.

Sodium sulfate: Na⁺ and SO₄²⁻ → Na₂SO₄ (one sulfate, no brackets needed).

Calcium hydroxide: Ca²⁺ and OH⁻ → two hydroxides → Ca(OH)₂. Without brackets, CaOH₂ would wrongly suggest one O and two H.

Aluminium sulfate: Al³⁺ and SO₄²⁻ → 2 Al³⁺ (+6) and 3 SO₄²⁻ (−6) → Al₂(SO₄)₃.

Ammonium carbonate: NH₄⁺ and CO₃²⁻ → (NH₄)₂CO₃.

Iron(III) nitrate: Fe³⁺ and NO₃⁻ → Fe(NO₃)₃.

Copper(II) hydrogencarbonate: Cu²⁺ and HCO₃⁻ → Cu(HCO₃)₂.

Part 2: covalent (molecular) compounds

Compounds of two non-metals are named with Greek prefixes that tell you the number of each atom directly — no charge balancing needed (see how to name covalent compounds).

Prefix	Number
mono-	1
di-	2
tri-	3
tetra-	4
penta-	5
hexa-	6

“Mono-” is usually left off the first element.

Worked examples

• Carbon dioxide → C, O₂ → CO₂
• Carbon monoxide → CO
• Dinitrogen tetroxide → N₂O₄
• Sulfur trioxide → SO₃
• Phosphorus pentachloride → PCl₅
• Diphosphorus pentoxide → P₂O₅ (the actual molecule is P₄O₁₀, but P₂O₅ is the traditional formula)
• Sulfur hexafluoride → SF₆
• Dinitrogen monoxide → N₂O (laughing gas)

Don’t simplify covalent formulas: N₂O₄ stays N₂O₄, not NO₂ — they’re different substances.

Common names to learn

Some covalent compounds are almost always known by common names: water (H₂O), ammonia (NH₃), methane (CH₄), hydrogen peroxide (H₂O₂), hydrogen chloride (HCl), hydrogen sulfide (H₂S).

Part 3: acids

Acids contain H⁺ combined with an anion. The number of H atoms equals the anion’s charge (see how to name acids).

• hydro-…-ic acid → from an -ide anion: hydrochloric acid → Cl⁻ → HCl(aq).
• …-ic acid → from an -ate anion: sulfuric acid → SO₄²⁻ → H₂SO₄.
• …-ous acid → from an -ite anion: nitrous acid → NO₂⁻ → HNO₂.

Part 4: hydrates

Write the salt formula, then a dot, then the number of water molecules from the Greek prefix (see how to name hydrates):

• Copper(II) sulfate pentahydrate → CuSO₄ + 5H₂O → CuSO₄·5H₂O
• Magnesium sulfate heptahydrate → MgSO₄·7H₂O

Part 5: diatomic elements

Seven elements exist as diatomic molecules when uncombined, so their formulas have a subscript 2:

H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂

A memory aid: they form a “7” shape on the periodic table starting at element 7 (nitrogen), plus hydrogen. Remember this when writing equations: “oxygen gas” is O₂, not O.

A worked walk-through: iron(III) sulfate

Let’s put the whole process together for a harder name.

1. Type? “Iron” is a metal, so this is ionic.
2. Cation? The Roman numeral III tells you the ion is Fe³⁺.
3. Anion? Sulfate is a polyatomic ion, SO₄²⁻.
4. Balance. The lowest common multiple of 3 and 2 is 6. Two Fe³⁺ ions give +6; three SO₄²⁻ ions give −6.
5. Write. Two iron ions, three sulfate ions, with brackets because there’s more than one sulfate: Fe₂(SO₄)₃.
6. Check. 2 × (+3) + 3 × (−2) = 0. ✓

The same six steps work for any ionic name, however long it looks. Most errors happen at step 3 (the wrong anion) or step 5 (a missing bracket), so slow down at those points.

Checking your formula

1. Charges balance (ionic): total + equals total −.
2. Lowest ratio (ionic): Mg₂O₂ should be simplified to MgO. (But don’t simplify covalent formulas or polyatomic ions — O₂²⁻, the peroxide ion, stays as it is.)
3. Brackets around polyatomic ions when there’s more than one.
4. Capital letters: each element symbol starts with a capital (Co is cobalt; CO is carbon monoxide).
5. Diatomic elements written with a 2.

You can check formulas, and their molar masses, with the molar mass calculator.

Common mistakes

• Using the number from the name as a charge in covalent compounds, or charge-balancing covalent names. Prefixes give numbers directly.
• Forgetting brackets: Ca(OH)₂, not CaOH₂.
• Changing polyatomic ions: sulfate is always SO₄²⁻; you can’t change it to SO₂ to “balance”.
• Wrong capitalisation: NaCl, not NACL or nacl.
• Confusing -ide, -ite and -ate: sulfide (S²⁻), sulfite (SO₃²⁻), sulfate (SO₄²⁻).

Practice

Write formulas for:

1. potassium bromide
2. barium nitrate
3. iron(II) hydroxide
4. ammonium phosphate
5. nitrogen triiodide
6. carbon tetrachloride
7. phosphoric acid
8. sodium carbonate decahydrate

Answers: 1 KBr · 2 Ba(NO₃)₂ · 3 Fe(OH)₂ · 4 (NH₄)₃PO₄ · 5 NI₃ · 6 CCl₄ · 7 H₃PO₄ · 8 Na₂CO₃·10H₂O

Key takeaways

• Decide the type: ionic, covalent, acid or hydrate.
• Ionic: find ion charges and balance them; bracket polyatomic ions when there’s more than one.
• Covalent: read the Greek prefixes directly; don’t simplify.
• Acids: anion + H⁺ to match the charge; hydrates: salt · n H₂O.
• Remember the seven diatomic elements.

For lots more practice, try naming compounds practice.