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460 BCE
Democritus
Democritus was born in 460 BC and died in 370 BC. Democritus expanded the atomic theory of Leucippus. He supposed the atoms, which are originally similar, to be impenetrable and have a density proportionate to their volume. Also, he claimed that the worlds which we see -- with all their properties of immensity, resemblance, and dissimilitude -- result from the endless multiplicity of falling atoms, and that the human soul consists of globular atoms of fire, -
John Dalton
John Dalton was born in 1766 and died in 1844. He based his
atomic theory of partial pressures on the idea that only like atoms in a mixture of gases repel one another, whereas unlike atoms appear to react indifferently toward each other. Dalton claimed that atoms of different elements vary in size and mass, and indeed this claim is the cardinal feature of his atomic theory. -
Dmitri Mendeleev
Dmitri Mendeleev was born in 1834 and died in 1907. Mendeleev found that, when all the known chemical elements were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of properties within groups of elements. -
Eugen Goldstein
Eugen Goldstein was born in 1850 and died in 1930. Goldstein discovered protons with the experiments he did with cathode rays which would knock electrons of atoms and attract them to a positively charged electrode. He also discovered several important properties of the cathode rays which contributed to their later identification as the electron. -
JJ Thomson
JJ Thomson was born in 1856 and died in 1940. Thomson determined that all matter is made up of tiny particles that are much smaller than atoms. He called these particles 'corpuscles,' and they are now called electrons. This discovery reversed the theory that the atom was the smallest fundamental unit. he discovered that neon was composed of two different kinds of atoms, -
Max Planck
Max Planck was born in 1858 and died in 1947. Planck's theory was about a particle. These particles were later called photons. In 1913, Niels Bohr used Planck's theories to develop a new and more accurate model of the atom. -
Robert Millikan
Robert Millikan was born in 1868 and died in 1953. As a scientist, Millikan made numerous momentous discoveries, chiefly in the fields of electricity, optics, and molecular physics. His earliest major success was the accurate determination of the charge carried by an electron, using the elegant “falling-drop method”; he also proved that this quantity was a constant for all electrons (1910), thus demonstrating the atomic structure of electricity. -
Ernest Rutherford
Ernest Rutherford was born in 1871 and died in 1937. In 1910, his investigations of the nature of the inner structure of the atom led to the postulation of his concept of the “nucleus”, his greatest contribution to physics. According to him practically the whole mass of the atom and at the same time all positive charge of the atom is concentrated in a minute space at the centre. -
Niels Bohr
Niels Bohr was born in 1885 and died in 1962. In Niels Bohr's theory of the atom, electrons absorb and emit radiation of fixed wavelengths when jumping between fixed orbits around a nucleus. The theory provided a good description of the spectrum created by the hydrogen atom, but needed to be developed to suit more complicated atoms and molecules. -
Erwin Schrodinger
Erwin Schrodinger was born in 1887 and died in 1961. Assuming that matter (e.g., electrons) could be regarded as both particles and waves, in 1926 Erwin Schrödinger formulated a wave equation that accurately calculated the energy levels of electrons in atoms. -
James Chadwick
In 1932, Chadwick made a fundamental discovery in the domain of nuclear science: he proved the existence of neutrons – elementary particles devoid of any electrical charge. In contrast with the helium nuclei (alpha rays) which are charged, and repelled by the considerable electrical forces present in the nuclei of heavy atoms, this new tool in atomic disintegration need not overcome any electric barrier and is capable of penetrating and splitting the nuclei of even the heaviest elements. -
Werner Heisenberg
Werner Heisenberg was born in 1901 and died in 1976. In 1925, Werner Heisenberg formulated a type of quantum mechanics based on matrices. In 1927 he proposed the "uncertainty relation", setting limits for how precisely the position and velocity of a particle can be simultaneously determined in the atom.
