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200
340 BC - Aristotle
ARISTOTLE disbelieved in the atomic theory that Democritus created nearly 100 years earlier. Instead, he thought that all materials on earth existed of not atoms, but of the "four elements": earth, air, water, and fire. As he was a major philosopher of the era, many people believed in the theory of Aristotle, yet Democritus' theory has since been proven more scientifically legitimate. -
200
490 BC - Democritus
DEMOCRITUS provided the initial atomic theory. He prophesized that "the universe is composed of two elements: the atoms and the void in which they exist and move." Democritus' theory consisted of 5 basic principles, which were: (1.) All matter consists of invisible particles called atoms. (2.) Atoms are indestructible. (3.) Atoms are solid but invisible.v(4.) Atoms are homogenous. (5.) Atoms differ in size, shape, mass, position, and arrangement. -
Antoine Lavoisier
ANTOINE LAVOISIER discovered that the mass of any given substance is consereved in a chemical reaction. In multiple experiments conducted by Lavoisier, he recorded that the mass of the reactants was ALWAYS equal to that of the products. We now better recognize this theory as the Law of Conservation of Mass. This proved to be a revolutionary discovery as it depened the understanding of how atoms and compounds of atoms react together. -
John Dalton
JOHN DALTON discovered that all elements are made up of atoms. His atomic theory consisted of 4 basic principles that established the general informamtion of the atom. Dalton's atomic theory contradicted the theory of Democritus, as it disproved the possibilty that all kinds of matter are alike. -
Henri Becquerel
HENRI BECQUEREL discovered radioactivity, the spontaneous emission of radiation by a material.He also proved that the radiation emitted by uranium shared certain characteristics with x-rays, yet could be deflected by a magnetic field and therefore must consist of charged particles. This greatly enhanced the way atoms and elements were explored and tested as, contrary to prior belief, atoms could be broken down into more elementary particles. -
JJ Thomson
JJ THOMSON discovered the electron, a sub-atomic particle. Thomson's model, unlike future models that would eventually prove his incorrect, illustrates a positively charged core with negatively charged particles within it. His model was also musch more massive than models that preceded his own. While his model might not have been fully correct based on current knowledge, Thomson's model (created before the discovery of the atomic nucleus.was the first model to illustrate a core of an atom. -
Marie & Perrie Curie
The CURIES developed te elements Polonium and Radium. They are probably most famous however, for their joint discovery of radioctivity, with the likes of their counterpart Henri Becquerel. This reveloutionized the way people interpreted the atom as prior to this discovery, scientists regarded the atom as the most elementary particle. Marie Curie is very closely associated with Becquerel as Curie based her doctoral thesis off of Becquerel's experiments. -
Max Planck
MAX PLANCK developed the quantum theory of energy. This theory states that energy is radiated in very minute and discrete quantized amounts or packets, rather than in a continuous unbroken wave. This very much developed the broad understanding of the atom as it was now known how the behavior of the particles and waves realted on an atomic scale. This theory was further applied in major experiments by Niels Bohr and Albert Einstein to develop further advanced models of the atom. -
Robert Millikan
ROBERT MILLIKAN accurately discovered the charge of the electron. He also determined that the same negative quantity was true for all electrons. Millikan's discoveries enhanced the model of the atom as people now knew that all electrons carried a negative charge, meaning the charge of atoms could now be calculated, and charged isotopes were thus discovered. -
Ernest Rutherford
ERNEST RUTHERFORD's model of the atom illustrated the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative particles circulate at some distance, much like planets revolving around the Sun. After an experiment using aplha rays, Rutherford concluded that Thomson's previous model was incorrect as there has to be a dense nucleus in the center of the atom, in which the all mass was concetrated. -
Henry Moseley
HENRY MOSELEY discovered that the atomic number of the atom is equal to that of the number of protons in the atom's nucleus. This refined the theory of Rutherford, as he theorized that the sum of the nucleons (not solely protons) was equal to the atomic number. This proved as the first significant scientific evidence however, that proved the theory of Niels Bohr aside from the Hydrgoen atom. -
Niels Bohr
NIELS BOHR developed a plausible explanation of atomic structure that corresponded with the atom's place in the periodic table of elements. His atomic model has atoms built up of successive orbital shells of electrons. This very much enhanced earlier models as it not only represents a central nucleus containing nucleons and surrounding electrons, but it also represents how the values of these particles corresponded with their placemnt in the periodic table. -
Werner Heisenberg
WERNER HEISENBERG formulated the quantum theory based on the use of matrices. This further enchanced Bohr’s original quantum theory, which gave a good description of the spectrum of the hydrogen atom, for more complicated atoms and molecules. Coincidentally, Bohr later met up with Heisenberg in 1941 to discuss nuclear research and theoretical physics. -
Erwin Schrodinger
ERWIN SCHRODINGER's model of the atom greatly enhanced the Bohr model, in terms of how the electron is represented. Unlike Bohr’s model, in which the only information that was important was the size of the orbit, which was described by the n quantum number, Schrodinger's model assumes that the electron is a wave, allowing the electron to occupy three-dimensional space. It therefore required three coordinates, or three quantum numbers, to describe the orbitals in which electrons can be found.
