When was the idea of an atom first developed? Describe this theory.

The existence of an atom was first suggested by Democritus, the idea is then developed in 1803 by John Dalton.

According to Dalton's atomic theory, matter is made up of atoms which are inseparable and can not be destroyed further. For a given element, all atoms have identical mass and properties. Atoms of two different elements combine to form compounds and rearrangement of these atoms results in chemical reaction.