Writing Chemical Formula of ionic Compounds (O level Chemistry)

Writing Chemical Formula of ionic Compounds (O level Chemistry)

Writing chemical formula and naming chemical compounds are some of the things O level chemistry students find challenging. This post is all about addressing the challenges faced by students when writing chemical formula and naming chemical compounds.


Naming binary ionic compounds

Binary ionic compounds contain only 2 elements, a metal and a nonmetal, in which the metal has lost all valence electrons to form a cation and the non-metal has gain electrons to form an anion.

When naming binary ionic compounds we start with the name of the metal, followed by the name of the nonmetal, ending in -ide. For example:

MetalNon-metalCompound formed
SodiumNitrogenSodium nitride
PotassiumOxygenPotassium oxide
ZincSulphurZinc sulphide
CalciumFluorineCalcium fluoride
MagnesiumIodineMagnesium iodide
SodiumChlorineSodium chloride
ZincPhosphorusZinc phosphide

Valence of an element

When writing the chemical formula of an ionic compound we first need to determine the valencies of the ions in the compound.


The valence of an element is the charge its atoms take when it loses or gains electrons to become an ion.

To determine to valence of an element we need to know either its proton number or its position on the periodic table.

Steps for writing chemical formula of binary ionic compounds

  • Write the names of the elements in the compound
  • Write the symbols of the two elements.
  • Write the valence of each as superscripts without the positive and negative signs.
  • Interchange the valencies and then write them as subscripts.
  • Reduce the ratio of the formula as simple as possible.

Chemical formula of sodium nitride

StepMetalNon-metal
Write the names of the elements in the compoundSodiumNitrogen
Write the symbols of the two elementsNaN
Write their valencesNa+N3-
Remove the signsNa1N3
Exchange the valences and make them subscriptsNa3N1

Therefore the formula of sodium nitride is Na3N.

Chemical formula of potassium oxide

StepMetalNon-metal
Write the names of the elements in the compoundPotassiumOxygen
Write the symbols of the two elementsKO
Write their valencesK+O2-
Remove the signsK1O2
Exchange the valences and make them subscriptsK2O1

Therefore the formula of potassium oxide is K2O.

Chemical formula of zinc sulphide

StepMetalNon-metal
Write the names of the elements in the compoundZincSulphur
Write the symbols of the two elementsZnS
Write their valencesZn2+S2-
Remove the signsZn2S2
Exchange the valences and make them subscriptsZn2S2
Reduce the ratio on the formulaZn1S1

Therefore the formula of zinc sulphide is ZnS

Chemical formula of compounds of transition metals

Just like regular metals, transition metals form ionic compounds with non-metals. Transition metals are elements that have variable valence. Since these metal do not have a fixed valence, their valences are stated in the names of their compounds. For example:

  • copper (I) oxide – is an oxide of copper in which copper has a valence of 1.
  • copper (II) oxide – is an oxide of copper in which copper has a valence of 2.
  • iron (II) oxide – is an oxide of iron in which iron has a valence of 2.
  • iron (III) oxide – is an oxide of iron in which iron has a valence of 3.

Chemical formula of copper (I) oxide

The valence of copper here is 1.

StepMetalNon-metal
Write the names of the elements in the compoundCopperOxygen
Write the symbols of the two elementsCuO
Write their valencesCu+O2-
Remove the signsCu1O2
Exchange the valences and make them subscriptsCu2O1

Therefore the formula of copper (I) oxide is Cu2O.

Chemical formula of copper (II) oxide

The valence of copper here is 2.

StepMetalNon-metal
Write the names of the elements in the compoundCopperOxygen
Write the symbols of the two elementsCuO
Write their valencesCu2+O2-
Remove the signsCu2O2
Exchange the valences and make them subscriptsCu2O2
Reduce to simplest ratioCu1O1

Therefore the formula of copper (II) oxide is CuO.

Chemical formula of iron (II) oxide

The valence of iron here is 2.

StepMetalNon-metal
Write the names of the elements in the compoundIronOxygen
Write the symbols of the two elementsFeO
Write their valencesFe2+O2-
Remove the signsFe2O2
Exchange the valences and make them subscriptsFe2O2
Reduce to simplest ratioFe1O1

Therefore the formula of iron (II) oxide is FeO.

Chemical formula of iron (III) oxide

The valence of iron here is 3.

StepMetalNon-metal
Write the names of the elements in the compoundIronOxygen
Write the symbols of the two elementsFeO
Write their valencesFe3+O2-
Remove the signsFe3O2
Exchange the valences and make them subscriptsFe2O3

Therefore the formula of iron (III) oxide is Fe2O3.

Chemical formula of vanadium (V) oxide

The valence of vanadium here is 5.

StepMetalNon-metal
Write the names of the elements in the compoundVanadiumOxygen
Write the symbols of the two elementsVO
Write their valencesV5+O2-
Remove the signsV5O2
Exchange the valences and make them subscriptsV2O5

Therefore the formula of vanadium (V) oxide is V2O5.

Steps for writing chemical formula of ionic compounds with polyatomic anions.

A polyatomic anion exists as a group of elements with a collective negative charge. Examples of polyatomic anions are:

  • sulphate ions, SO42-
  • sulphite ions, SO32-
  • nitrate ions, NO3
  • nitrite ions, NO2
  • carbonate ions, CO32-
  • hydroxyl ions, OH
  • ammonium ions, NH4+
  • phosphate ions, PO43-

When writing the chemical formula of a compound that contains a polyatomic ion, we follow the following steps:

  • Write the names of the elements and ions in the compound
  • Write the symbols of the ions involved.
  • Write the valence of each as superscripts without the positive and negative signs.
  • Interchange the valencies and then write them as subscripts.
  • Reduce the ratio of the formula as simple as possible.

Chemical formula of copper (II) sulphate

The valence of copper here is 2.

StepMetalNon-metal
Write the names of the elements and ionsCopperSulphate
Write the chemical symbols and the valencesCu2+SO42-
Remove the signsCu2SO42
Exchange the valences and make them subscriptsCu2(SO4)2
Reduce to simplest ratioCu1(SO4)1

Therefore the formula of copper (II) sulphate is CuSO4

Chemical formula of copper (II) hydroxide

The valence of copper here is 2.

StepMetalNon-metal
Write the names of the elements and ionsCopperHydroxide
Write the chemical symbols and the valencesCu2+OH
Remove the signsCu2OH1
Exchange the valences and make them subscriptsCu1(OH)2

Therefore the formula of copper (II) hydroxide is Cu(OH)2.

When you have more than one of a specific polyatomic ion in a formula you must enclose it in parenthesis.

Sir Sydney
Author: Sir Sydney

Tutor at Sytech Learning Academy.

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Sir Sydney

Tutor at Sytech Learning Academy.