Topic 1 of 6
Read and construct formulae
Subscripts count atoms; ion charges determine ratios.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Read and construct formulae
Subscripts count atoms; ion charges determine ratios.
An element symbol begins with a capital letter; a second letter is lower-case. Co is cobalt, while CO is carbon monoxide. In H2SO4, one formula unit contains 2 H, 1 S and 4 O atoms. Brackets multiply everything inside: Ca(OH)2 contains 1 Ca, 2 O and 2 H. A coefficient multiplies the entire formula: 3H2O represents six H atoms and three O atoms.
Worked example
From atom counts to a formula
A model molecule contains two nitrogen atoms and four oxygen atoms. What is its formula, and how many oxygen atoms are in three molecules?
- Write N and O with the atom counts as subscripts: N2O4.
- Three molecules contain 3 x 4 = 12 oxygen atoms. Do not simplify N2O4 to NO2 when the actual atoms in one molecule are specified.
N2O4; 12 oxygen atoms in three molecules. A coefficient counts complete molecules; a subscript counts atoms within one.
| Family | Symbols or formulae |
|---|---|
| Common non-metals | H, C, N, O, F, P, S, Cl, Br, I; elemental gases H2, N2, O2, Cl2 |
| Metals and other elements | Li, Na, K, Mg, Ca, Al, Zn, Fe, Cu, Ag, Pb; Si, He, Ne, Ar |
| Acids, alkalis and gases | HCl, HNO3, H2SO4, NaOH, Ca(OH)2, NH3, CO, CO2, SO2, NO, NO2, O3 |
| Common salts and reagents | NaCl, MgCl2, CaCO3, CuSO4, AgNO3, Ba(NO3)2, KI, KMnO4 |
| Oxides and organic examples | CaO, MgO, CuO, SiO2; CH4, C2H4, C2H5OH, CH3COOH |
An ionic formula must have zero total charge. Al3+ and O2- balance at +6 and -6, giving Al2O3. Mg2+ and NO3- give Mg(NO3)2. Keep a polyatomic ion intact; use brackets only when more than one is needed.
| Positive ions | Negative ions |
|---|---|
| Na+, K+, Ag+, NH4+ | Cl-, Br-, I-, OH-, NO3- |
| Mg2+, Ca2+, Zn2+, Cu2+, Fe2+, Pb2+ | O2-, CO32-, SO42- |
| Al3+, Fe3+ | A Roman numeral gives a metal ion oxidation state; distinguish iron(II) from iron(III). |