Valency

What is valency?

Atoms join to make compounds. Valency is a number that tells how many bonds one atom can make. It is the combining power of the atom.

The number comes from the outer shell (valence shell). The electrons there are the valence electrons. Atoms give, take or share them to join with other atoms.

We write valency in Roman numerals: I, II, III, IV, and so on. Example: in water, oxygen joins with two hydrogen atoms, so oxygen has valency II and hydrogen has valency I.

Valency from the periodic table: groups 1, 2 and 13

A group is a vertical column of the periodic table. Atoms in one group have the same number of outer electrons, so they have the same valency.

• Group 1 (Li, Na, K): 1 outer electron, valency I.
• Group 2 (Be, Mg, Ca): 2 outer electrons, valency II.
• Group 13 (B, Al): 3 outer electrons, valency III.

Rule: for groups 1 and 2 the valency is the group number. For group 13 it is the group number minus 10.

Valency from the periodic table: groups 14 to 17

Take the last digit of the group number: group 14 gives 4, group 15 gives 5, group 16 gives 6, group 17 gives 7. This is also the number of outer electrons.

• With oxygen, the highest valency equals this digit: C is IV, N and P are V, S is VI, Cl is VII.
• With hydrogen, the valency is 8 minus the digit: C is IV (CH₄), N is III (NH₃), O is II (H₂O), Cl is I (HCl).

Why 8? A full outer shell holds 8 electrons. The atom needs the missing ones, so it makes that many bonds with hydrogen.

Some non-metals show more than one valency. For example sulfur can be IV (SO₂) or VI (SO₃). The question tells you which one to use, often with a Roman numeral in the name.

Writing the formula of an oxide from valency

An oxide is a compound of an element with oxygen. Oxygen has valency II.

Steps for the formula:

1. Write the symbol and valency of the element and of oxygen: Al (III), O (II).
2. Swap the numbers (criss-cross): Al gets 2, O gets 3.
3. Write Al₂O₃.
4. If both numbers can be divided by the same number, divide. Mg (II) and O (II) swap to Mg₂O₂, which becomes MgO.

The hook picture explains it: total hooks of the metal atoms must equal total hooks of the oxygen atoms. 2 Al give 6 hooks and 3 O give 6 hooks.

Examples: Na₂O, MgO, Al₂O₃, CO₂, SiO₂, P₂O₅, SO₃, Cl₂O₇.

Naming oxides and finding valency from a formula

Name an oxide as element name + oxide: sodium oxide (Na₂O), magnesium oxide (MgO), aluminium oxide (Al₂O₃). If an element has more than one oxide, show the valency with a Roman numeral: sulfur(IV) oxide (SO₂) and sulfur(VI) oxide (SO₃). The prefixes di-, tri-, penta- are also common: carbon dioxide, diphosphorus pentoxide.

Going backwards (formula to valency): the number on oxygen is the valency of the other element split over its atoms. In P₂O₅, 5 oxygen atoms bring 10 hooks, so 2 phosphorus atoms share 10 hooks and each has valency V. In Fe₂O₃: 3 × 2 = 6 hooks, shared by 2 Fe, so iron is III.

Try it

Predict first, then check in the 3D. Pick Ca with oxygen: what formula do you expect? Then pick P with oxygen: how many oxygen atoms are needed for 2 phosphorus atoms? On paper, draw hooks for N with hydrogen and count how many hydrogen atoms fit. Compare with NH₃.

Key formulas and definitions

• Groups 1, 2: valency = group number (I, II)
• Group 13: valency = group number − 10 = III
• Groups 14–17, with oxygen: valency = last digit of group (IV to VII)
• Groups 14–17, with hydrogen: valency = 8 − last digit
• Criss-cross: X(m) and Y(n) give X_n Y_m, then divide by the common factor
• Oxygen has valency II; hydrogen has valency I

Worked examples

Common mistakes

• Forgetting to reduce the formula: Mg2O2 must be written MgO.
• Using the group number for groups 13 to 17 directly. Use the last digit: group 13 gives 3, group 17 gives 7.
• Mixing up the two rules for groups 14 to 17: the group digit is for oxygen, 8 minus the digit is for hydrogen.
• Changing the small numbers inside a formula to balance hooks. The valency stays; only the number of atoms changes.