Atom
Atoms are the smallest particles of an element which can take part in a chemical reaction.
Size of an atom: atomic radius is measured in nanometres.
The atomic symbol has three parts: -
The symbol X: the usual element symbol
The atomic number A: equal to the number of protons
The mass number Z: equal to the total number of protons and neutrons in an element.
                                                      Atomic Mass

Atomic mass and atomic mass unit

Atomic mass is the total of the masses of the electrons, neutrons, and protons in an atom, or in a group of atoms, the average mass.
Mass of an atomic particle is called the atomic mass.
This is commonly expressed as per the international agreement in terms of a unified atomic mass unit (AMU).
It can be best defined as 1/12 of the mass of a carbon -12 atom in its ground state.
Molecule

It is the smallest particle of an element or a compound which can exist independently.
•    Molecules of an element constitute the same type of atoms.
•    Molecules may be monoatomic, diatomic or polyatomic.
•    Molecules of compounds join together in definite proportions and constitute a different type of atoms.