Valency – IGCSE Chemistry Definition and How to Use It

Valency is the single concept that allows candidates to write correct formulae without memorising hundreds of compounds. Paper 2 and Paper 4 regularly ask for the formula of a compound given the ions or the valencies, and the cross-over method takes seconds once learned.

The 0620 definition

Valency is the combining power of an element — the number of chemical bonds an atom can form, or equivalently, the number of electrons an atom uses in bonding (by sharing or transferring). For ions, valency equals the size of the charge.

Valency from the periodic table

For main-group elements, valency follows a pattern across the periods:

Group	Valency	Examples
I	1	Na, K, Li — form 1+ ions or one bond
II	2	Mg, Ca, Ba — form 2+ ions or two bonds
III	3	Al, B — form 3+ ions or three bonds
IV	4	C, Si — form four covalent bonds
V	3	N, P — form three bonds (or have valency 5 in some compounds)
VI	2	O, S — form two bonds or 2- ions
VII	1	F, Cl, Br, I — form one bond or 1- ions
0	0	He, Ne, Ar — do not bond

Transition elements can have variable valency. Iron, for example, can be Fe²⁺ (valency 2) or Fe³⁺ (valency 3). The valency is shown by a Roman numeral in the name: iron(II) chloride = FeCl₂, iron(III) chloride = FeCl₃.

The cross-over method for writing formulae

To write the formula of a compound from two elements or ions:

1. Write the symbols side by side.
2. Write each element’s valency above its symbol.
3. Cross the valencies: each valency becomes the subscript of the other element.
4. Simplify to the lowest whole-number ratio.

Example: aluminium oxide

Al has valency 3, O has valency 2.

Cross over: Al₂O₃. Already in lowest ratio. Formula: Al₂O₃.

Example: calcium chloride

Ca has valency 2, Cl has valency 1.

Cross over: CaCl₂. Formula: CaCl₂.

Example: magnesium oxide

Mg has valency 2, O has valency 2.

Cross over: Mg₂O₂. Simplify: MgO. Formula: MgO.

Valency of common polyatomic ions

Some ions contain more than one element but act as a single unit with a fixed valency:

Ion	Formula	Valency
Hydroxide	OH⁻	1
Nitrate	NO₃⁻	1
Carbonate	CO₃²⁻	2
Sulfate	SO₄²⁻	2
Ammonium	NH₄⁺	1
Phosphate	PO₄³⁻	3

When a polyatomic ion needs a subscript greater than 1, place it in brackets:

Calcium hydroxide: Ca(OH)₂ — not CaOH₂ or CaO₂H₂.

Aluminium sulfate: Al₂(SO₄)₃ — not Al₂S₃O₁₂ (though numerically equivalent, the bracket notation preserves the identity of the sulfate ion).

Linking valency to electron configuration

Valency connects directly to electron configuration:

• Metals in Groups I–III lose their outer electrons to form positive ions with charges equal to the group number. Their valency equals the number of electrons lost.
• Non-metals in Groups V–VII gain electrons to complete their outer shell, or share electrons to form covalent bonds. Their valency equals the number of electrons needed to fill the shell (8 minus the group number).
• Group IV elements share four electrons, forming four covalent bonds (e.g. carbon in methane, CH₄).

Common exam mistakes

1. Forgetting to simplify. Mg₂O₂ must be written as MgO. Na₂S₂ must be written as NaS — wait, Na has valency 1 and S has valency 2, so the cross-over gives Na₂S₁, which is Na₂S. Always check.
2. Not using brackets for polyatomic ions. Ca(OH)₂ is correct; CaOH₂ implies the 2 applies only to the H.
3. Confusing valency with group number for non-metals. Chlorine is in Group VII but has valency 1 (it needs one electron), not 7.
4. Ignoring variable valency of transition metals. If the question says “iron(III)”, the valency is 3, giving FeCl₃ with chlorine, not FeCl₂.