An atom is the smallest particle of a chemical element that can exist.

A molecule is the smallest particle of an element or of a compound which can exist freely under ordinary conditions, and shows all the properties of that substance. A molecule may contain one, two, three or more atoms of the same or different kinds. Molecules are characterized by the number of atoms present in it.
An atom is a smallest particle of matter which may or may not have independent existence. Egs.- C,P,Mg,K,Al etc..

Molecules are the smallest particle of a substance that exist freely under ordinary conditions. A molecule can have two or more atoms of an element. Eg. Hydrogen molecule contains 2 atoms of hydrogen, Sulphur molecule contains 8 atoms of sulphur.