Atom is a Greek word and its meaning is indivisible i.e. an ultimate particle which cannot be further subdivided.
John Dalton considered that "all matter are composed of smallest indivisible particle called atom."
Daltons Atomic Theory :
This theory is based on law of mass conservation and law of definite proportions. The salient features of this theory are :-
(1) Each element is composed of extremely small particles called atoms.
(2) Atoms of a particular element are like but differ from atoms of other element.
(3) Atom of each element is an ultimate particle and it has a characteristic mass but is structureless
(4) Atoms are indestructible i.e. they can neither be created nor be destroyed.
(5) Atoms of different elements take part in chemical reaction to form molecule.
Next topic - Discovery of Subatomic Particles