Atomic Theory

Events

Democritus 400 BC

Approx. 1300

Democritus’ atomic theory posited that all matter is made up small indestructible units he called atoms.

Isaac Newton

1704

Isaac Newton theorized a mechanical universe with small, solid masses in motion.

John Dalton

1803

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

Michael Faraday

1832

Michael Faraday developed the two laws of electrochemistry.

J. Plucker

1859

J. Plucker built one of the first cathode-ray tubes.

Dmitri Mendeleev

1869

Dmitri Mendeleev created the periodic table.

James Clerk Maxwell

1873

James Clerk Maxwell proposed the theory of electromagnetism and made the connection between light and electromagnetic waves.

G.J. Stoney

1874

G.J. Stoney theorized that electricity was comprised of negative particles he called electrons.

Sir William Crookes

1879

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

Eugen Goldstein

1886

Eugen Goldstein discovered canal rays, which have a positive charge equal to an electron.

Wilhelm Roentgen

1895

Wilhelm Roentgen discovered x-rays.

Henri Becquerel

1896

Henri Becquerel discovered radiation by studying the effects of x-rays on photographic film.

Joseph John Thomson

1897

Joseph John Thomson determined the charge to mass ratio of electrons.

Marie Sklodowska Curie

1898

Marie Sklodowska Curie discovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium.

Ernest Rutherford

1898

Ernest Rutherford discovered alpha, beta, and gamma rays in radiation.

Frederick Soddy

1900

Frederick Soddy came up with the term "isotope" to explain the unintentional breakdown of radioactive elements.

Max Planck

1900

Max Planck proposed the idea of quantization to explain how a hot, glowing object emitted light.

Hantaro Nagaoka

1903

Hantaro Nagaoka proposed an atomic model called the Saturnian Model to describe the structure of an atom.

Richard Abegg

1904

Richard Abegg found that inert gases have a “stable electron configuration.”

Hans Geiger

1906

Hans Geiger invented a device that could detect alpha particles.

H. G. J. Moseley

1914

Henry Gwyn Jeffreys Moseley discovered that the number of protons in an element determines its atomic number.

Francis William Aston

1919

Francis William Aston used a mass spectrograph to identify 212 isotopes.

Niels Bohr

1922

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

Louis de Broglie

1923

Louis de Broglie proposed that electrons have a wave/particle duality.

John Douglas Cockcroft and Ernest Thomas Sinton Walton

1929

John Douglas Cockcroft and Ernest Thomas Sinton Walton created the first nuclear reaction, producing alpha particles

Paul Dirac

1930

Paul Dirac proposed the existence of anti-particles.

James Chadwick

1932

James Chadwick discovered neutrons, particles whose mass was close to that of a proton.

Otto Hahn, Lise Meitner, and Fritz Strassmann

1938

Otto Hahn, Lise Meitner, and Fritz Strassmann discovered nuclear fission.

Glenn Seaborg

1941 - 1951

Glenn Seaborg discovered eight transuranium elements.

Enrico Fermi

1942

Enrico Fermi created the first man-made nuclear reactor.