Atom

The Atomic Theory

  • 460 BCE

    Democritus

    One of the first atomic theorists was Democritus, a Greek philosopher who lived in the fifth century BC. Democritus knew that if a stone was divided in half, the two halves would have essentially the same properties as the whole.Therefore, he reasoned that if the stone were to be continually cut into smaller and smaller pieces then; at some point, there would be a piece which would be so small as to be indivisible. He called these small pieces of matter "atomos," the Greek word for indivisible.
  • 322 BCE

    Aristotle

    Aristotle did not believe in the atomic theory and he taught so otherwise. He thought that all materials on Earth were not made of atoms, but of the four elements, Earth, Fire, Water, and Air. He believed all substances were made of small amounts of these four elements of matter. Aristotle's view was finally proven incorrect and his teachings are not present in the modern view of the atom.
  • Isaac Newton

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

    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

    Michael Faraday developed the two laws of electrochemistry.
  • J. Plucker

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

    built one of the first cathode-ray tubes.
  • Dmitri Mendeleev

    created the periodic table.
  • Dmitri Mendeleev

    created the periodic table
  • James Clerk Maxwell

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

    proposed the theory of electromagnetism and made the connection between light and electromagnetic waves.
  • G.J. Stoney

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

    theorized that electricity was comprised of negative particles he called electrons.
  • 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.
  • 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.
  • E. Goldstein

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

    discovered canal rays, which have a positive charge equal to an electron.
  • Wilhelm Roentgen

    discovered x-rays.
  • Wilhelm Roentgen

    discovered x-rays.
  • Henri Becquerel

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

    discovered radiation by studying the effects of x-rays on photographic film.
  • J.J. Thomson

    determined the charge to mass ratio of electrons.
  • J.J. Thomson

    determined the charge to mass ratio of electrons.
  • Rutherford

    discovered alpha, beta, and gamma rays in radiation
  • Marie Sklodowska Curie

    discovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium.
  • Rutherford

    discovered alpha, beta, and gamma rays in radiation.
  • Marie Sklodowska Curie

    discovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium.
  • Max Planck

    proposed the idea of quantization to explain how a hot, glowing object emitted light.
  • Frederick Soddy

    came up with the term "isotope" to explain the unintentional breakdown of radioactive elements.
  • Max Planck

    proposed the idea of quantization to explain how a hot, glowing object emitted light.
  • Frederick Soddy

    came up with the term "isotope" to explain the unintentional breakdown of radioactive elements.
  • Hantaro Nagaoka

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

    proposed an atomic model called the Saturnian Model to describe the structure of an atom.
  • Richard Abegg

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

    found that inert gases have a “stable electron configuration.”
  • Hans Geiger

    invented a device that could detect alpha particles.
  • Hans Geiger

    invented a device that could detect alpha particles.
  • H.G.J. Moseley

    discovered that the number of protons in an element determines its atomic number.
  • H.G.J. Moseley

    discovered that the number of protons in an element determines its atomic number.
  • Francis William Aston

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

    used a mass spectrograph to identify 212 isotopes.
  • Niels Bohr

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

    proposed that electrons have a wave/particle duality.
  • Cockcroft / Walton

    created the first nuclear reaction, producing alpha particles
  • Paul Dirac

    proposed the existence of anti-particles.
  • James Chadwick

    discovered neutrons, particles whose mass was close to that of a proton.
  • Lise Meitner, Hahn, Strassman

    discovered nuclear fission.
  • Glenn Seaborg

    discovered eight transuranium elements.
  • Enrico Fermi

    created the first man-made nuclear reactor.