Atom is the smallest independent existing particle.

Molecule is formed from two or more atoms.

Element- A substance that cannot be broken down into simpler substances by chemical means. An element is composed of atoms that have the same atomic number i.e. each atom has the same number of protons in its nucleus as all other atoms of that element.

Compound- A substance consisting of atoms or ions of two or more different elements in definite proportions joined by chemical bonds into molecule. Eg. Water has two hydrogen and one oxygen. Therefore it is a compound.