Atomic Theory

Democritus (BC)

400

Located in Greece, he proposed that all matter including space and time is made up of miniscule unbreakable units called atoms. These atoms are always in motion and differ according to shape, arrangement, and position. This contributed to our understanding of the atom because he created the original Atomic Theory.
Sourced from: http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec05.html

Newton

Jan 1, 1704

Located in England, he proposed that there is a mechanical universe with small solid masses in motion. He also came up with a description of universal gravity by constructing the three laws of motion. This contributed to our understanding of the atom because he laid the base for differential and integral calculus. Sourced from:
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Dalton

Jan 1, 1803

Located in England, he proposed that there are spherical solid atoms with measurable properties of mass. This contributed to our understanding of the atom because he helped with the development of the modern Atomic Theory. Sourced from:
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G. J. Stoney

Jan 1, 1894

Located in Ireland, he proposed that electricity was made up of separate negative particles called electrons.This contributed to our understanding of the atom because he came up with the idea of electrons. Sourced from:
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J. J. Thomson

Jan 1, 1897

Located in United Kingdom, he conducted an experiment using a CRT (Cathort Ray Tube) and proposed that the charge to mass ratio (e/m) of an electron is 1.759 x 108 coulombs/gram.This contributed to our understanding of the atom because he showed that an atom was in fact made of smaller parts that all fit together. Sourced from:
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Planck

Jan 1, 1900

Located in Germany, he proposed an explanation of hot glowing matter by using the idea of separate units of energy.
This contributed to our understanding of the atom because he showed the movement of electrons. Sourced from:
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Nagaoka

Jan 1, 1903

Located in Japan, he proposed a model of an atom and based it on the planet Saturn. He predicted that electrons revolved around the nucleus and were bound by electrostatic forces and that it had a large nucleus. It also showed electrons revolving around a positively charged particle.This contributed to our understanding of the atom because he was the first to show the structure of an atom. Sourced from:
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Millikan

Jan 1, 1909

Located in U.S.A, he conducted an experiment (the Oil drop experiment) that determined the charge (e=1.602 x 10 -19 coulomb) and the mass (m = 9.11 x 10 -28 gram) of an electron. He established that the weight of an electron is 1836 times smaller than that of a hydrogen atom (atomic mass of 1). This contributed to our understanding of the atom because of the accuracy of his results ( they were exact figures). Sourced from:
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E. Rutherford

Jan 1, 1911

Located in the United Kingdom and New Zealand, he proposed that the nucleus was very dense, small and positively charged and that electrons were located outside the nucleus. He did so by using alpha particles as atomic bullets and probing the atoms in a piece of thin (0.00006 cm) gold foil. This contributed to our understanding of the atom because he visually showed an atom (the nucleus, protons, neutrons and electrons). Sourced from:
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H. G. J. Moseley

Jan 1, 1914

Located in England, he proposed the charges on the nuclei of most atoms by using x-ray tubes in an experiment. He said the atomic number of an element is equal to the number of protons in the nucleus.This contributed to our understanding of the atom because his experiment lead to the reorganisation of the periodic table based upon atomic number instead of atomic mass. Sourced from:
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Bohr

Jan 1, 1922

Located in Denmark, he proposed that electrons had a duel nature just like particles and waves and travelled only in certain orbits. He also said that different electron shells have the capacity to hold different numbers of electrons.This contributed to our understanding of the atom because his atomic model explained the underlying regularities of the periodic table of elements. Sourced from:
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James Chadwick

Jan 1, 1932

Located in England, he discovered the third subatomic neutral particle, the neutron. He used alpha particles to discover this particle with a mass close to a proton.This contributed to our understanding of the atom because he came up with the idea of neutrons. Sourced from:
http://atomictimeline.net/index.php