What do you understand by atom and molecule?

Atom: Atom is the smallest particle of an element which may or may not have independent existence. For example, the atoms of hydrogen are not capable of independent existence while the atoms of neon gas are capable of independent existence. Atoms take part in chemical combination and remain as indivisible.

Molecule: Molecule is the smallest particle of a substance (element or compound) which is capable of independent existence. The molecules are formed by the combination of two or more atoms. In case the atoms happen to be the same, the molecules are called homoatomic molecules (e.g. O₂, N₂, H₂ etc.). In case atoms happen to be different, the molecules are known as heteroatomic molecules (e.g. NH₃, CO₂, SO₂ etc.).