Chemical formulae

• A chemical formula of a compound represents the composition of a molecule of the substance in terms of the symbols of the elements present in the molecule.

Formulae of Elements:

• The chemical formula of an element is a symbolic representation of element the subscript if present tells about the number of atoms present in the molecule.

For example:

H₂  2 atoms of Hydrogen in one molecule

N₂  2 atoms of Nitrogen in one molecule

O₂  2 atoms of Oxygen in one molecule

Cl₂  2 atoms of Chlorine in one molecule

Br₂  2 atoms of Bromine in one molecule

I₂ 2  atoms of Iodine in one molecule

P₄  4 atoms of Phosphorous in one molecule

P₈  8 atoms of Phosphorous in one molecule

Formulae of compounds:

• The chemical formula of a compound is a symbolic representation of compound which tells about the number of atoms of each element present in the compound.

For example:

H₂  2 atoms of Hydrogen in one molecule

N₂  2 atoms of Nitrogen in one molecule

O₂  2 atoms of Oxygen in one molecule

Cl₂  2 atoms of Chlorine in one molecule

Br₂  2 atoms of Bromine in one molecule

I₂ 2  atoms of Iodine in one molecule

P₄  4 atoms of Phosphorous in one molecule

P₈  8 atoms of Phosphorous in one molecule

Writing chemical formula of a molecular compound:

Step 1:

Write the symbols of the elements present in the compound.

Example:

Aluminium chloride

Aluminium [Al]                 Chlorine [Cl]

Step 2:

Write the valency of each element.

Al  valency 3    and   Cl valency  1   (Cross placement of valency)