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Democritus
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The theory of Democritus held that everything is composed of "atoms", which are physically, but not geometrically, indivisible; that between atoms, there lies empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, -
John Dalton
Dalton being led by his search for an explanation of the law of multiple proportions to the idea that chemical combination consists in the interaction of atoms of definite and characteristic weight, the idea of atoms arose in his mind as a purely physical concept, forced upon him by study of the physical properties of the atmosphere and other gases -
Joeseph John Thompson
Thomson discovered the electron in the year 1897. His work put forward a new theory, that atom was made up of small particles.Thus he discovered the electrons. He proved his theory using the cathode ray tube. -
Ernest Ruthford
He discovered alpha and beta rays, set forth the laws of radioactive decay, and identified alpha particles as helium nuclei. Most important, he postulated the nuclear structure of the atom -
Niels Henrik David Bohr
In 1913, Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities -
Current Atomic Theroy
Bohr's model of the atom is important because it introduced the concept of the quantum in explaining atomic properties. However, Bohr's model ultimately needed revision becuase it failed to explain the natue of atoms more complicated than hydrogen. It took roughly another decade before a new more complete atomic theory was developed - the modern atomic theory.