Atomic Theory

Democritus

400

That all matter is made up small indestructible units he called atoms.

Isaac Newton

Dec 7, 1704

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

John Dalton

Dec 7, 1803

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

J. Plucker

Dec 7, 1859

Built one of the first cathode-ray tubes.

Demitri Mendeleev

Dec 7, 1869

Created the periodic table.

G.J Stoney

Dec 7, 1874

Theorized that electricity was comprised of negative particles he called electrons.

Sir William Crookes

Dec 7, 1879

Experiments with cathode-ray tubes led him to confirm the work of earlier scientists by definitively demonstrating that cathode-rays have a negative charge.

E. Goldstein

Dec 7, 1886

Discoered canal rays, which have a positive charge equal to an electron.

J.J Thomson

Dec 7, 1897

Determined the charge to mass ratio of electrons.

Marie Curie

Dec 7, 1898

Discovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium

Frederick Soddy

Dec 7, 1900

Ccame up with the term "isotope" to explain the unintentional breakdown of radioactive elements

Hantaro Nagaoka

Dec 7, 1903

Proposed an atomic model called the Saturnian Model to describe the structure of an atom.

Hans Gieger

Dec 7, 1906

Invented a device that could detect alpha particles.

H.G.J Moseley

Dec 7, 1914

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

Francis William Aston

Dec 7, 1919

Uused a mass spectrograph to identify 212 isotopes.

Niels Bohr

Dec 7, 1922

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

Louis De Broglie

Dec 7, 1923

Proposed that electrons have a wave/particle duality.

Cockcroft/Walton

Dec 7, 1929

Created the first nuclear reaction, producing alpha particles

James Chadwick

Dec 7, 1932

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

Lise Meitner/Hans/Strassman

Dec 7, 1938

Discovered nuclear fission

Glenn Seaborg

Dec 7, 1941

Discovered eight transuranium elements.

Enrico Fermi

Dec 7, 1942

Created the first man-made nuclear reactor.