How to write a chemical formula ?

To write a chemical equation, the reactants should be written on the left, and the products should be written on the right and the coefficients should be written next to the symbols of entities to indicate the number of moles of a substance produced used in the chemical reaction.

→ A Chemical formula represents the proportions, by mass of the different elements in a compound.

→ A binary compound is composed of two different elements only. Formulae of binary compounds can be written using valencies since both types of atom forming the compound must lose, gain or share the same number of electrons.

• To write the chemical formula of a binary compound.

Determine the valencies of the two elements present.

Write the chemical formula such that the sums of the valencies of the two elements are equal. Do this by determining the lowest common multiple of the two valencies.

If a metal is present, always place it first in the formula.

Solved Examples

1. Aluminium oxide

Valencies of elements:

Al = 3 (Group III)

O = 2 (Group VI)

Lowest common multiple of valencies = 6

Al = 2 x 3 = 6

O = 3 x 2 = 6

Therefore the chemical formula = Al2O3