Chemistry : Writing Chemical Formulae for Chemical Compounds

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How to write a Chemical Formula for a Compound?

A chemical formula is the representation of a substance by symbols. More importantly, it denotes the number of atoms of each element present in the compound. For example, the formula for Ferric oxide or Iron [III] oxide is Fe₂O₃, which implies that 2 atoms of Fe and 3 atoms of O are present in an electrically-neutral molecule of the compound. To write a chemical formula, one must know the symbols and valencies of the elements / radicals.

Example 1. Write the chemical formula for Calcium hydroxide.
Valency of Calcium (Ca) = 2 ; Valency of Hydroxide (OH) = 1.
Interchanging their valencies and writing as subscripts (ignoring 1),
formula for Calcium hydroxide is Ca(OH)₂.
Note that 1 calcium ion [Ca²⁺] and 2 hydroxide ions [OH^1-] are present in an electrically-neutral molecule of calcium hydroxide [Ca(OH)₂].

Example 2. Write the chemical formula for Magnesium sulfate.
Valency of Magnesium (Mg) = 2 ; Valency of Sulfate (SO₄) = 2.
Interchanging their valencies and simplifying (on dividing by 2),
formula for Magnesium sulfate is MgSO₄.
Note that 1 magnesium ion [Mg²⁺] and 1 sulfate ion [SO₄^2-] are present in an electrically-neutral molecule of magnesium sulfate [MgSO₄].

Points to remember:

• When the subscript is 1, it is ignored.
• The radical is written in parenthesis when the subscript is 2 or greater.
• Whenever possible, subscripts are simplified by dividing by the highest common factor (HCF).

Practice Exercise for Chemistry Module on Chemical Formulae of Compounds - Basic

Practice Exercise for Chemistry Module on Chemical Formulae of Compounds-Advanced

Practice Exercise for Chemistry Module on Common Chemical Compounds