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324
Aristotle
He denied the existence of atoms; he was denial was unchallenged for 2,000 years simply because Democritus had only ideas - no research or proof for his theory of atoms. -
370
Democritus
He claimed that matter was made up of atoms. Atoms could not be destroyed, created or separated. His theory of the nature of the physical world was the most radical and scientific attempted up to his time. -
Period: 370 to 370
Democritus
He claimed that matter was made up of atoms -
John Dalton
He was the first to develop the modern atomic theory; he conducted research and expanded to Democritus's ideas. He is best known as the weather pioneer who advocated atomic theory through weather observations. -
Period: to
John Dalton
He was the first to develop the modern atomic theory; he conducted research and expanded on Democritus's ideas -
Dmitri Mendeleev
He created a table of the known elements (63 at the time) in ascending order by atomic weight, grouped together by similarities in properties. -
Sir William Crookes
He experimented with and confirmed the work of earlier scientists. He demonstrated that the cathode rays have a negative charge. He also is uncredited with the discovery of the electron because he did not record his findings. -
Ernest Rutheford
Rutheford developed the first logical explanation of the structure of an atom. He showed that the atom consists of a central, positively charged core, the nucleus, and negatively charged particles called electrons that orbit the nucleus -
Niels Bohr
Bohr said that electrons could circle a nucleus without radiating oenergy only in orbits for which their orbital angular momentum was a multiple of Planck's constant h divided by 2n. -
H.G.J. Mosely
He showed that each nucleus was characterized by an atomic number; and that equals the number of protons. -
Robert Millikan
He measured the charge on the electron and is known for his work on the photoelectric effect. He made important studies of cosmic rays, X rays, and physical and electric constants and wrote and lectured on the reconciliation of science and religion. -
Erwin Schrödinger
He is known for his mathematical development of wave mechanics, a form of quantum mechanics, and his formulation of the wave equation (The Schrödinger equation). He received the 1933 Nobel Prize in Physics. -
James Chadwick
James Chadwick discovered the neutron. He worked at Manchester under Ernest Rutheford on radioactivity. He received the 1935 Nobel Prize in Physics. -
J.J. Thomson
He determined the charge to mass ratio of electrons He is credited with the discovery of electrons and isotopes. He was also awarded the 1906 Nobel Prize in Physics for the discovery of the electron and for his work on the conduction of electricity in gases.
