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John Dalton
Gave the idea of an Atomic Theory with, based on measurable mass properties, spherical solid atoms. -
Dmitri Mendeleev
Group elements with similarities into 7 groups. Found that the properties of the elements periodic functions of their atomic weights. -
J.J. Thompson
Determined the charge of mass ratio: (e/m) of an electron=1.759 x 10 8, using a CRT. -
Ernest Rutherford
Found that electrons were located outside the nucleus; which was dense, small, and positively charged. (not sure about specific date) -
Niels Bohr
Explained that atomic structure underlies regularities of the periodic table of elements. (not sure about specific date) -
Enrico Fermi
Done the first controlled chain reaction which released energy from the atoms nucleus.