Valency

Valency

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.

Radical

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
MetalsPotassiumCalciumAluminium
SodiumMagnesium
SilverBarium
Zinc
Non MetalsHydrogenOxygenNitrogen
ChlorineSulphurPhosphorus
RadicalsHydroxideOxidePhosphate
ChlorideCarbonate
NitrateSulphate
NitriteSulphite

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
Valencies32
Exchange23

Formula: Al2O3

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

StepsComponent 1Component 2
Atom/ RadicalCuOH
Valencies21
Exchange12

Formula: Cu(OH)2

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

StepsComponent 1Component 2
Atom/ RadicalMgO
Valencies22
Exchange22

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