Matter is made up of discrete particles namely, atoms, molecules and ions.
An atom is the smallest particle of an element which can take part in a chemical reaction. Examples are C, O and H.
A molecule is the smallest part of a substance that can exist independently. It is formed by a chemical combination of two or more atoms. Examples are Cl2, H2O and H2.
An ion is a substance that is made up of charged particles. Examples are Na+, Cl-.
Dalton’s Atomic Theory
John Dalton, a British chemist, studied the atom extensively and proposed the theory that:
i) Matter is made up of small indivisible particles called atoms.
ii) Atoms of the same element are alike but differ from those of all other elements.
iii) Chemical changes involve separation and recombination of atoms.
iv) Atoms can neither be created nor be destroyed .