Chemical Formulae

Before a single mole can be calculated, the formula has to be right, and Cambridge knows it, which is why “write the formula of aluminium sulfate” style questions sit in Papers 1-4 as 1-mark gatekeepers. Get Al2(SO4)3 wrong and any calculation built on it collapses. Formula writing is Core, mechanical, and learnable in an evening; this page gives the complete method that the stoichiometry pillar compresses into a paragraph.

The ion charges to memorise (Core)

Positive ions	Charge	Negative ions	Charge
Group I (Na+, K+)	+1	Group VII (Cl−, Br−, I−)	−1
Group II (Mg2+, Ca2+)	+2	Group VI (O2−, S2−)	−2
Group III (Al3+)	+3	Group V (N3−)	−3
Ammonium NH4+	+1	Hydroxide OH−, nitrate NO3−	−1
Silver Ag+, zinc Zn2+	+1, +2	Carbonate CO3 2−, sulfate SO4 2−	−2

Transition metals show their charge in Roman numerals in the name: iron(II) means Fe2+, iron(III) means Fe3+, copper(II) means Cu2+. The Roman numeral is information, not decoration.

Swap-and-drop (Core)

To write an ionic formula: write both ions with charges, then use each ion’s charge number as the other ion’s subscript, and simplify. Calcium chloride: Ca2+ and Cl− → CaCl2. Aluminium oxide: Al3+ and O2− → Al2O3. When a compound ion needs multiplying, bracket it: magnesium nitrate is Mg(NO3)2; ammonium sulfate is (NH4)2SO4. Check the result is neutral: in Al2(SO4)3, 2 × (+3) balances 3 × (−2).

Molecular (covalent) formulae are not derived from charges; they are learned: H2O, CO2, NH3, CH4, SO2, plus the diatomic elements H2, N2, O2, F2, Cl2, Br2, I2. A formula also has a meaning to state: it tells you which elements are present and the ratio of atoms (or, for molecules, the actual number of each atom in one molecule).

Balancing symbol equations (Core)

A balanced equation has the same number of each kind of atom on both sides, because atoms are never created or destroyed in a reaction. The only adjustable numbers are coefficients, the large numbers in front of complete formulae. Method: write correct formulae first (never alter them afterwards), count each element across both sides, then balance one element at a time, leaving lone elements (O2, H2, metals) until last.

Example: methane burning. CH4 + O2 → CO2 + H2O. Carbon balances; hydrogen needs 2H2O; oxygen now needs 2O2. Final: CH4 + 2O2 → CO2 + 2H2O. State symbols complete the picture when requested: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l). Extended candidates also write ionic equations (covered step-by-step in our ionic equations and balancing guide), and the coefficients you fix here become the mole ratios in reacting-mass calculations.

Worked exam question

Iron(III) oxide reacts with dilute sulfuric acid to form iron(III) sulfate and water. (a) Write the formula of iron(III) sulfate. (1) (b) Write the balanced symbol equation for the reaction. (2)

Model answer: (a) Fe2(SO4)3 (1). (b) Fe2O3 + 3H2SO4 → Fe2(SO4)3 + 3H2O, correct formulae throughout (1); correct balancing (1).

Mark-by-mark: (a) needs the bracket and both subscripts. Fe3+ and SO4 2− swap to give 2 and 3. In (b) the first mark is for every formula being right, the second for the 1:3:1:3 coefficients; one wrong formula usually forfeits both, because the equation can no longer balance correctly. Writing FeSO4 (iron(II)) instead shows the Roman numeral was ignored.

The mistakes that cost marks

1. Changing subscripts to balance: turning H2O into H2O2 to fix an oxygen count invents hydrogen peroxide. Coefficients only.
2. Dropping brackets: MgNO32 instead of Mg(NO3)2. Without the bracket the formula reads as one nitrogen and 32 oxygens, an automatic zero.
3. Ignoring Roman numerals: copper(I) oxide is Cu2O, copper(II) oxide is CuO. The numeral sets the charge, the charge sets the formula.
4. Forgetting diatomic elements: writing H, O or Cl as free elements in equations. Hydrogen gas is H2, and balancing done with lone atoms collapses.

How examiners want it phrased

Typical student wording	Accepted mark-scheme wording
”I made the numbers equal on both sides"	"The equation is balanced: 1 C, 4 H and 4 O on each side"
"Aluminium sulfate = AlSO4"	"Al3+ and SO4 2− give Al2(SO4)3"
"Add a 2 to the H2O"	"Place the coefficient 2 in front of H2O: 2H2O"
"Solid means s"	"State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water)“

The Malaysia note

Malaysian schools following the two-year IGCSE track teach formula writing early in Year 10, and by the May/June exam season it is assumed knowledge inside electrolysis, salts and organic questions and is never retaught. Students from SPM-aligned backgrounds know the same swap method (“kaedah silang”), so transfers adapt fast. When a student’s calculation marks look weak, wrong formulae are the cause about as frequently as wrong mole logic; the diagnostic in our free 1-hour trial separates the two in minutes, because the fixes are completely different.

Test yourself

Write all three answers down first. They stay hidden until you click.

Q1 (2 marks). Write the formula of calcium nitrate and of ammonium carbonate.

Show answer

• Calcium nitrate: Ca2+ and NO3− give Ca(NO3)2 [1] • Ammonium carbonate: NH4+ and CO3 2− give (NH4)2CO3 [1]

Q2 (2 marks). Balance the equation for the complete combustion of propane: C3H8 + …O2 → …CO2 + …H2O

Show answer

• 3CO2 and 4H2O (balancing carbon, then hydrogen) [1] • 5O2, giving C3H8 + 5O2 → 3CO2 + 4H2O [1]

Q3 (2 marks). Write the formula of iron(II) hydroxide and of copper(I) oxide.

Show answer

• Iron(II) hydroxide: Fe2+ and OH− give Fe(OH)2 [1] • Copper(I) oxide: Cu+ and O2− give Cu2O [1]