Common ions and formulae

Almost every chemical formula in SPM Chemistry is built from a small set of ions with fixed charges. Once you know the charge on each common ion, writing a formula becomes mechanical: you balance the total positive and negative charge so the compound is neutral. This reference lists the ions the syllabus expects and then shows the combining rule, arranged the way the DSKP presents it, so you can build any salt formula with confidence.

Scope note

The two tables below hold the standard KSSM set of common ions for Form 4 and Form 5. Learn the name, the symbol and the charge together, because all three are needed to write a formula and to name a salt correctly. These are the ions that appear in formula writing, ionic equations and qualitative analysis.

Common cations (positive ions)

Name	Formula	Charge
Hydrogen	H+	1+
Sodium	Na+	1+
Potassium	K+	1+
Silver	Ag+	1+
Ammonium	NH4+	1+
Copper(II)	Cu2+	2+
Iron(II)	Fe2+	2+
Iron(III)	Fe3+	3+
Zinc	Zn2+	2+
Lead(II)	Pb2+	2+
Magnesium	Mg2+	2+
Calcium	Ca2+	2+
Barium	Ba2+	2+
Aluminium	Al3+	3+

Common anions (negative ions)

Name	Formula	Charge
Hydroxide	OH-	1-
Chloride	Cl-	1-
Bromide	Br-	1-
Iodide	I-	1-
Nitrate	NO3-	1-
Hydrogencarbonate	HCO3-	1-
Ethanoate	CH3COO-	1-
Oxide	O2-	2-
Sulfide	S2-	2-
Sulfate	SO4 2-	2-
Carbonate	CO3 2-	2-
Nitride	N3-	3-
Phosphate	PO4 3-	3-

The combining rule

To write a formula, balance the charges so the compound is neutral. The quick method is to swap the size of each charge to become the subscript of the other ion, then simplify. For sodium oxide, Na⁺ and O²⁻ combine as Na₂O; for aluminium oxide, Al³⁺ and O²⁻ combine as Al₂O₃. When you need more than one polyatomic ion, put it in brackets: calcium nitrate is Ca(NO₃)₂, and aluminium sulfate is Al₂(SO₄)₃. Always check the final ratio is in whole numbers with no common factor left.

Worked reasoning

Write the formula of ammonium sulfate. The ammonium ion is NH₄⁺ (1+) and the sulfate ion is SO₄ 2- (2-). To balance the 2- charge you need two ammonium ions, so the formula is (NH₄)₂SO₄. The brackets show that everything inside is doubled, and the total charge is now 2+ against 2-, giving a neutral compound. Practising a few of these until they are automatic makes equation writing much faster.

Naming salts from the two tables

The same tables let you name a salt as well as write its formula. A salt name is simply the cation followed by the anion, with the anion ending telling you the acid it came from: chloride from hydrochloric acid, sulfate from sulfuric acid, and nitrate from nitric acid. So a compound of copper(II) and sulfate is copper(II) sulfate, written CuSO₄, and a compound of ammonium and chloride is ammonium chloride, NH₄Cl. When a metal can form more than one ion, the Roman numeral in the name fixes the charge you must use, which is why iron(II) chloride is FeCl₂ while iron(III) chloride is FeCl₃. Reading name to formula and back until both directions are fluent is one of the most reliable ways to save time in the exam.

Common mistakes to avoid

• Forgetting the charge on a polyatomic ion, such as writing sulfate as SO₄⁻ instead of SO₄ 2-.
• Leaving out brackets, so Ca(NO₃)₂ is wrongly written as CaNO₃₂.
• Confusing iron(II) and iron(III); the Roman numeral gives the charge.
• Not simplifying, for example writing Mg₂O₂ instead of MgO.

How we help

Our online one-to-one teachers drill formula writing from this ion set until it becomes second nature, because every equation you balance depends on it. Lessons are in English, one to one, from RM50 per hour, with a paid one-hour trial. Secure ion charges early and the mole chapter, ionic equations and salt preparation all become noticeably easier through Form 4 and Form 5.