Atom
An atom is the smallest particle of an element that takes part in chemical reactions and maintains its chemical identity throughout all chemical and physical changes. Atoms of one element are different from the other elements. Free atoms except those of the noble gases; do not exist under the normal conditions.
Molecule
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 (element or compound). 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.