Atomic Theory Research

Democritus 400BC (Greece)

400

All matter is made up small indestructible units he called atoms.

Newton (England)

Jan 1, 1704

Theorized a mechanical universe with small, solid masses in motion.

Dalton (England)

Jan 1, 1803

That elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together.

G.J. Stoney (Ireland)

Jan 1, 1894

That electricity was comprised of negative particles he called electrons.

J.J. Thomson (U.K.)

Jan 1, 1897

Determined the charge to mass ratio of electrons.

Planck (Germany)

Jan 1, 1900

Proposed the idea quantization to explain how a hot, glowing object emitted light.

Nagaoka (Japan)

Jan 1, 1903

Proposed an atomic model called the Saturnian Model to describe the structure of the atom. It was used for him to show that he predicted that an atom had a very massive nucleus and there are electrons revolving around the nucleus, bound by electrostatic forces.

Milikan (U.S.A)

Jan 1, 1909

Accurate determination of the charge carried by an electron, using the elegant "falling-drop method." He also proved that this quantity was a constant for all electrons.

E. Rutherford (New Zealand)

Jan 1, 1911

Theorized that atoms are constructed much like the solar system. That is, a heavy part, called the nucleus, forms the centre of each atom. Particles with a negative charge, called electrons, form the outer part, most of which consists of an empty space. Discovered alpha, beta, and gamma rays in radiation.

H.G.J. Mosely (England)

Jan 1, 1914

Discovered that the number of protons in an element determines its atomic number.

Bohr (Denmark)

Jan 1, 1922

Proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.

James Chadwick (England)

Jan 1, 1932

Discovered neutrons, particles whose mass was close to that of a proton.