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History of Atomic Theory

  • Period: to

    Milestones in Atomic Theory

  • Newton-The suggestion that atoms were held together by forces

    He defines how small particles can affect each other across a distance by the act of forces, also helping in the understanding of co-valent and ionic compounds.
  • Dalton-Researched and defined the main points of atoms

    We now know that elements are made of tiny particles called atoms, atoms cannot be created or divided, atoms in a given element are identical and atoms in different elements can be distinguished by their relative atomic weight.
  • G.J. Stoney-The conception and calculation of the magnitude of the "atom of electricity"

    He proposed the term “electron”, to describe the essential unit of electrical charge. This laid foundation for the discovery of the particle later on.
  • J.J Thompson-Studied “canal rays” and experimented with electrons.

    He discovered through his studies that atoms were in fact made up of other things (protons and electrons). Secondly, he determined the charge to mass ratio of an electron (=1.759 x 10 8 coulombs/gram).
  • Planck-Used the idea of quanta (discrete units of energy) to explain hot, glowing matter.

    He deduced the relationship between energy and the frequency of radiation and had the idea that the energy emitted by a resonator could only take on discrete values or quanta.
  • Nagaoka-Proposed a Saturn-like model of the atom with rings of electrons surrounding a positively charged particle.

    He provided the knowledge that electrons revolve around the atom in rings, bound by electrostatic forces.
  • Millikan-Through an oil-drop experiment, determined the charge carried by an electron.

    He provided the knowledge of the charge on an electron and that this charge is constant for all electrons, thus demonstrating the atomic structure of electricity.
  • E. Rutherford-He studied the atom and its nucleus in extreme detail.

    He concluded that the nucleus was very small, very dense and assumed that electrons were outside of it. He is also credited with discovering the proton.
  • H.G.J Mosely-Using X-Ray tubes, determined the charge on the nuclei of most atoms.

    He wrote that the atomic number of an element is equal to the number of protons in a nucleus. This information was used to reorganize the periodic table according to atomic number instead of atomic mass.
  • Bohr-Developed an explanation of atomic structure using his model which was an atom surrounding by successive electron shells.

    He proposed that each electron shell going out would have more electrons than the previous ones and that the outer shells determine the atom’s chemical properties.
  • James Chadwick-Founded the neutron

    Chadwick discovered a third part of the atom, which added weight to an atom but not charge. Hence, the neutron was discovered.