John Dalton is considered the father of Modern Atomic theory. According to Dalton, matter is made up of tiny particles called atoms. The atom is the smallest particle of matter that takes part in a chemical reaction. Atoms are indivisible and cannot be created or destroyed. According to this theory atoms can not be created, destroyed, or divided as they are the smallest particles of matter.
But later it was proved that atoms can be divided into sub atomic particles proton, neutron and electrons. These sub atomic particles can also take part in the reaction. This was proved with the discovery of nuclear fission and fusion.