Valency is a measure of the number of hydrogen atoms an atom or radical can combine with.

Hydrogen is the simplest known atom and has a valency of 1, so it is used as the standard.

Valency is also defined as the number of electrons lost or gained when an atom is bonding to form a compound. This therefore means that valency of an atom is its combining power.

Since only outer shell electrons are lost or gained during chemical bonding, these electrons are called valence electrons.


A radical is a group of atoms that exist in compounds but does not exist on its own.

Examples of radicals:

  1. SO4 – Sulphate radical that exists in compounds such as Sulphuric acid (H2SO4)
  2. NO3 – Nitrate radical that exists in compounds such as Copper Nitrate (Cu(NO3)2)

Valencies of Elements and Radicals

The valency of chlorine is 1 because a chlorine atom gains 1 electron when bonding.
The valency of oxygen is 2 because an oxygen atom gains 2 electrons when bonding.

Type of elementValency 1Valency 2Valency 3
Non MetalsHydrogenOxygenNitrogen

Variable valencies

Some elements have variable valencies. Their valencies vary depending on the conditions of the reaction when they combine.

For example:

  1. The valency of copper can either be 1 or 2.
  2. The valency of iron can either be 2 or 3.

The valency of substances like these is shown in roman numerals when writing names of their compounds.

For example:

  1. Iron (II) Chloride, FeCl2 – Iron has a valency of 2 there.
  2. Iron (III) Chloride, FeCl3 – Iron has a valency of 3 there.
  3. Copper (I) Oxide, Cu2O – Copper has a valency of 1 there.
  4. Copper (II) Oxide, CuO – Copper has a valency of 2 there.

Metals have a valency of 2, except:

  • Potassium, Sodium, Silver ➾ 1.
  • Aluminium, Iron(III) ➾ 3.

Writing chemical formulae using valencies

Its easy to write the chemical formula of a binary compound if you know the valencies of its components.

For example, lets deduce the formula of Sodium Sulphate.

  • Write the symbols of the elements and radicals in the compound:
  • Na SO4
  • Write the valencies above the symbols:
  • Na1 SO42.
  • Exchange the valencies, i.e. 1 from Na goes to SO4 and 2 from SO4 goes to Na. Then write those numbers below and after the symbol. If its 1 dont write anything.
  • Na2SO4.

Another example, deduce the chemical formula of Aluminium oxide.

StepsComponent 1Component 2
Atom/ RadicalAlO

Formula: Al2O3

Yet another example, deduce the chemical formula of Copper hydroxide.

StepsComponent 1Component 2
Atom/ RadicalCuOH

Formula: Cu(OH)2

Finally the final example, deduce the chemical formula of Magnesium Oxide.

StepsComponent 1Component 2
Atom/ RadicalMgO

2:2 means 1:1 which means the formula is MgO.