Molecule:A group of atoms bonded together, representing the smalllest fundamental unit of a chemical compound.It can also take part in a chemical reaction.Atomicity:Atomicity is the number of atoms present in the molecules of an element.Ion:An atom or molecule with a net electric charge due to loss or gain of electrons.

Molecule are bounded in with two are more atomsAtomicity is number of atoms present in a elementCompounds are composed with charged species, the charged species are known as ion

a) A molecule is a particle made up of two or more atoms that are chemically bonded together; the number of atomic nuclei making up a molecule is a determinate number. For example, HCl(g)is a molecule made of one hydrogen atom bonded to one chlorine atom. It is made of two atoms and is called a diatomic molecule.

b) Atomicity is the number of atoms of an element present in one molecule of that element. For example, there are 2 atoms of hydrogen present in H2 molecule so it atomicity is 2.

c) an electrically charged atom or group of atoms formed by the loss or gain of one or more electrons, as a cation (positive ion) , which is created by electron loss and is attracted to the cathode in electrolysis, or as an anion (negative ion) , which is created by an electron gain and is attracted to the anode.