CHEMICAL FORMULA

FORMULA

The  atomic composition of a water molecule can be decribed either 

in words , i.e.
'Each molecule of water contains two atoms of hydrogen and one atom of  oxygen chemically combined together.
or 
by its chemical formula, i.e.

                                    H2O
Obviously, the chemical formula is a smaller method of  showing the atomic composition of the water molecule.
The CHEMICAL FORMULA of a molecule shows the number and kinds of atoms contained in it. The chemical formula of a molecule of an element is denoted by the chemical symbol of the element and a subscript showing the number of atoms in the molecule. For example, H2 denotes a molecule of hydrogen which consists of two hydrogen atoms.

 The same method  is used in representing the chemical formulae of compounds. There are at least two chemical symbols in the chemical formula  of a compound because each molecule of a compound is made up of two or more different types of atoms. When more than one atom of an element is present, the number of atoms is given by a subscript. When no subscript is written, we assume that the number of atom is one.