Atom may be defined as the smallest particle of an element which does not exist free in nature but takes part directly in chemical combinations.
Atom of any element is represented by the symbol of that element. For example, hydrogen atom is represented by H, sodium atom is represented by Na, etc.
Molecule may be defined as the smallest particle of an element or compound which exists free in nature but does not participate directly in chemical combinations.
Molecule of any substance is represented by their molecular formula. For example, water molecule is represented by H2O. It tells that each water molecule contains two atoms of hydrogen and one atom of oxygen.
