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406
Democritus (406 BCE)
Democritus (Greek era 460 BCE) "by convention bitter, by convention sweet, but in reality atoms and void” Democritus of Abdera is best known for the atomic theory. Also the indivisible model. -
Antoine Lavoisier (1743-1794)
Lavoisier found that mass is conserved in a chemical reaction. The total mass of the products of a chemical reaction is always the same as the total mass of the starting materials consumed in the reaction. Also atoms are neither created nor destroyed in a chemical reaction. -
Joseph Louis Proust (1754-1826)
He first published his Law of Definite Proportions (or Law of Constant Composition) in 1794.This law states that a compound is composed of exact proportions of elements by mass regardless of how the compound was created. -
John Dalton (1776-1844) + Indivisible, solid sphere model
Dalton recognized the existence of atoms of elements and that compounds formed from the union of these atoms. He therefore assumed that simplest ratios would be used in nature and came up with a formula for water and then assigned a relative atomic weight of one to hydrogen and developed a relative atomic weight scale from percent composition data and assumed atomic ratios. Today we would refer to these as equivalent masses. -
Michael Faraday
Studied the effect of electricity on solutions, coined term "electrolysis" as a splitting of molecules with electricity, developed laws of electrolysis. Faraday himself was not a proponent of atomism. -
J.J. Thomson (1860-1940) + plum pudding model
J. J. Thomson identified the positive and negatively charged electron in the cathode ray tube in 1897 and deduced that the electron was a component of all matter and calculated the charge to mass ratio for the electron. The charge to mass ratio depended on filling gas in the tube. Thomson proposed the "plum pudding" model of the atom. -
Robert Millikan (1868-1953)
Robert Millikan determined the unit charge of the electron with his oil drop experiment in 1909 at the University of Chicago. Thus allowing for the calculation of the mass of the electron and the positively charged atoms. e = 1.60 x 10-19 coulombs -
Ernst Rutherford (1871-1937) + Electron Cloud Model
Rutherford proposed the nuclear atom as the result of the gold-foil experiment in 1911. Rutherford proposed that all of the positive charge and all of the mass of the atom occupied a small volume at the center of the atom and that most of the volume of the atom was empty space occupied by the electrons.Rutherford in 1920 was the first to refer to the hydrogen nucleus as a proton. Also in 1920, Rutherford proposed the existence of the third atomic particle, the neutron. -
Albert Einstein (1879-1955)
In 1916 proposes general theory of relativity-still central to our understanding of the universe. Einstein changed the twentieth century, through his scientific foundation in the development of atomic energy. E=mc² -
Henri Becquerel
Becquerel studied the effect of x-rays on photographic film. Becquerel discovered that certain chemicals spontaneously decompose and give off very penetrating rays. -
Marie and Pierre Curie
Studied uranium and thorium and called their spontaneous decay process "radioactivity". She and her husband Pierre also discovered the radioactive elements polonium and radium. -
Max Planck
Planck used the idea of quanta (discrete units of energy) to explain hot glowing matter. -
Niels Bohr + planetary model
Developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of successive orbital shells of electrons. -
Louis DeBroglie
Discovered that electrons had a dual nature-similar to both particles and waves. Particle/wave duality. -
Erwin Schrödinger + Quantum mechanical model
Schrödinger discovered that an electron in an atom would move as a wave, contrary to de Broglie's belief. The following year, he wrote a revolutionary paper that highlighted what would be known as the Schrödinger wave equation. -
James Chadwick
Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron. -
Lise Meitner (1878–1968)
Conducted experiments with Fritz Strassmann (1902–1980) and Otto Hahn in 1938 verifying that heavy elements capture neutrons and form unstable products which undergo fission. This process ejects more neutrons continuing the fission chain reaction. -
Otto Hahn (1879–1968)
Conducted experiments with Fritz Strassmann (1902–1980) and Lise Meitner in 1938 verifying that heavy elements capture neutrons and form unstable products which undergo fission. This process ejects more neutrons continuing the fission chain reaction. -
Glenn T. Seaborg (1941 - 1951)
Synthesized 6 transuranium elements and suggested a change in the layout of the periodic table. -
Aristotle (384-322 BCE)
He believed that regardless of the number of times you divide something it can always become smaller. He also believed that everything can be explained with the four elements air, earth, fire, and water.
