The smallest unit into which a substance can be divided without changing it\’s chemical nature
A group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction.
Atoms together joined my a bond is called Molecule
Molecule is made up of atoms.

A molecule is the smallest unit (particle) of a compound having the physical and chemical properties of that compound. This does not mean that molecules can not be broken down into smaller parts, e.g. the atoms from which they are formed or the fragments of the molecule, each consisting of several atoms or parts of atoms.

A\xa0molecule\xa0is the smallest particle in a chemical element or compound that has the chemical properties of that element or compound.\xa0Molecules\xa0are made up of atoms that are held together by chemical bonds. These bonds form as a result of the sharing or exchange of electrons among atoms.

The combination of atoms of same or different types of atoms in the fixed ration by mass is known as molecules.The combination of same types of atoms is called molecule of element e.g. N2 while different types of atom form molecule of compound e.g. H2O