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Democritus
Democritus was an ancient Greek philosopher who lived from 460 BC to 370 BC. He was an influential pre-Socratic philosopher and pupil of Leucippus, who formulated what is thought to be the first atomic theory. Some people consider him to be the father of modern science. It is hard to separate his theories from those of Leucippus, since they are always mentioned in the same texts, but their theories have very different basis. -
John Dalton
John Dalton (1766-1844) He proposed the Atomic theory of matter based on his experimental observations. The main postulates of Dalton’s atomic theory are as follows.
To Dalton : All matter is made of atoms. Atoms are indivisible and indestructible. All atoms of a given element are identical in mass and properties. Compounds are formed by a combination of two or more different kinds of atoms. A chemical eaction is a rearrangement of atoms. Atoms can be neither created nor destroyed. -
JJ Thomson
Thomson's atomic theory proposed a model of atom which is known as plum pudding model or Christmas pudding or chocolate chip cookie model. Till the end of the nineteenth century the concept of atom was similar to a small solid billiard ball. In the year 1897 Joseph John Thomson (1856–1940) totally changed the view of an atom by discovering electron. Thomson’s atomic theory suggested that the atom is not indivisible as it was of smaller pieces – electrons and protons. -
James Chadwick
In 1932, James Chadwick identified the neutron. The particle proposed by Rutherford as having significant mass and no charge. With the discovery of the neutron three subatomic particles were identified that would help explain observations made at the atomic level. One observation was the existence of radioactive variances of the same element. How could two atoms of the same element have identical chemical properties but one be radioactive and the other not?
