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:

Metal Non-metal Compound formed
Sodium Nitrogen Sodium nitride
Potassium Oxygen Potassium oxide
Zinc Sulphur Zinc sulphide
Calcium Fluorine Calcium fluoride
Magnesium Iodine Magnesium iodide
Sodium Chlorine Sodium chloride
Zinc Phosphorus Zinc 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

Step Metal Non-metal
Write the names of the elements in the compound Sodium Nitrogen
Write the symbols of the two elements Na N
Write their valences Na+ N3-
Remove the signs Na1 N3
Exchange the valences and make them subscripts Na3 N1

Therefore the formula of sodium nitride is Na3N.

Chemical formula of potassium oxide

Step Metal Non-metal
Write the names of the elements in the compound Potassium Oxygen
Write the symbols of the two elements K O
Write their valences K+ O2-
Remove the signs K1 O2
Exchange the valences and make them subscripts K2 O1

Therefore the formula of potassium oxide is K2O.

Chemical formula of zinc sulphide

Step Metal Non-metal
Write the names of the elements in the compound Zinc Sulphur
Write the symbols of the two elements Zn S
Write their valences Zn2+ S2-
Remove the signs Zn2 S2
Exchange the valences and make them subscripts Zn2 S2
Reduce the ratio on the formula Zn1 S1

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 โ€“ is an oxide of copper in which copper has a valence of 1.
  • copper (II) oxide โ€“ is an oxide of copper in which copper has a valence of 2.
  • iron (II) oxide โ€“ is an oxide of iron in which iron has a valence of 2.
  • iron (III) oxide โ€“ 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.

Step Metal Non-metal
Write the names of the elements in the compound Copper Oxygen
Write the symbols of the two elements Cu O
Write their valences Cu+ O2-
Remove the signs Cu1 O2
Exchange the valences and make them subscripts Cu2 O1

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

Chemical formula of copper (II) oxide

The valence of copper here is 2.

Step Metal Non-metal
Write the names of the elements in the compound Copper Oxygen
Write the symbols of the two elements Cu O
Write their valences Cu2+ O2-
Remove the signs Cu2 O2
Exchange the valences and make them subscripts Cu2 O2
Reduce to simplest ratio Cu1 O1

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

Chemical formula of iron (II) oxide

The valence of iron here is 2.

Step Metal Non-metal
Write the names of the elements in the compound Iron Oxygen
Write the symbols of the two elements Fe O
Write their valences Fe2+ O2-
Remove the signs Fe2 O2
Exchange the valences and make them subscripts Fe2 O2
Reduce to simplest ratio Fe1 O1

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


Chemical formula of iron (III) oxide

The valence of iron here is 3.

Step Metal Non-metal
Write the names of the elements in the compound Iron Oxygen
Write the symbols of the two elements Fe O
Write their valences Fe3+ O2-
Remove the signs Fe3 O2
Exchange the valences and make them subscripts Fe2 O3

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

Chemical formula of vanadium (V) oxide

The valence of vanadium here is 5.

Step Metal Non-metal
Write the names of the elements in the compound Vanadium Oxygen
Write the symbols of the two elements V O
Write their valences V5+ O2-
Remove the signs V5 O2
Exchange the valences and make them subscripts V2 O5

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, NO3โ€“
  • nitrite ions, NO2โ€“
  • carbonate ions, CO32-
  • hydroxyl ions, OHโ€“
  • 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.

Step Metal Non-metal
Write the names of the elements and ions Copper Sulphate
Write the chemical symbols and the valences Cu2+ SO42-
Remove the signs Cu2 SO42
Exchange the valences and make them subscripts Cu2 (SO4)2
Reduce to simplest ratio Cu1 (SO4)1

Therefore the formula of copper (II) sulphate is CuSO4


Chemical formula of copper (II) hydroxide

The valence of copper here is 2.

Step Metal Non-metal
Write the names of the elements and ions Copper Hydroxide
Write the chemical symbols and the valences Cu2+ OHโ€“
Remove the signs Cu2 OH1
Exchange the valences and make them subscripts Cu1 (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.