-
470
Democritus
Developed a theory that Matter was composed of tiny particles called atoms, that could bond with other atoms to create more. -
Period: 470 to
Development of Atomic Theory
-
Newtown
Proposed a mechanical universe with small solid masses in motion. -
Dalton
Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass. -
G. J. Stoney
Discovered that electricity was made up of negative particles called ‘electrons’. -
J. J. Thomson
The ‘Plum Pudding Theory’. -
Planck
Used the idea of quanta which is discrete units of energy, to explain hot glowing matter.
Created Planck’s constant and Planck’s Law of Black-body Radiation. -
Nagaoka
Created a ‘Saturnian’ or Planetary model of the atom, with flat rings representing electrons, revolving around a positively charged particle, the model representing electrons revolving around a nucleus which was bound by electrostatic forces. -
Millikan
Conducted the ‘Millikan Oil drop experiment ‘ which determined the charge (e=1.602 x 10 -19 coulomb) and the mass (m = 9.11 x 10 -28 gram) of an electron. -
E. Rutherford
Established that the nucleus was very dense, very small and positively charged. Also assumed that the electrons were located outside the nucleus. -
H.G.J. Moseley
Discovered each nucleus had an ‘atomic number’ that was equal to the number of positive charges (protons) associated. -
Bohr
Developed the ‘Shell model’ that had atoms built up of successive orbital shells of electrons. -
James Chadwick
Used alpha particles leading to the discovery of a neutral atomic particle with a mass close to a proton. Thus, the neutron was discovered.