Development Of The Atomic Model

By Abinesh R
  • Period: 460 BCE to 370 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.
  • Period: to

    Dalton

    Dalton proposed that:
    - All matter consists of tiny particles called atoms.
    - Atoms are indestructible and unchangeable.
    - Elements are characterized by the weight of their atoms.
    - In chemical reactions, atoms combine in small, whole-number
    ratios.
    - When elements react, their atoms may combine in more than
    one whole-number ratio.
  • Period: to

    Thomson

    In 1897, J.J. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He demonstrated that cathode rays were negatively charged. Thomson realized that the accepted model of an atom did not account for negatively or positively charged particles. Therefore, he proposed a model of the atom which he likened to plum pudding. The negative electrons represented the raisins in the pudding and the dough contained the positive charge.
  • Period: to

    Rutherford

    In 1911, Ernest Rutherford proved Thomson's plum pudding structure incorrect. Rutherford aimed alpha particles at solid substances such as gold foil and recorded the location of the alpha particle "strikes" on a fluorescent screen as they passed through the foil. To the experimenters’ amazement, although most of the alpha particles passed unaffected through the gold foil as expected, a small number of particles were deflected at an angle, and a few ricocheted straight back.
  • Period: to

    Bohr

    Bohr proposed that electrons are arranged in concentric circular orbits around the nucleus. His theory can be summarised by the principles that:
    - Electrons occupy only certain orbits around the nucleus.
    - Each orbit has an energy associated with it.
    - Energy is absorbed when an electron jumps from a lower orbit to a
    higher one.
    - The energy and frequency of light emitted or absorbed can be
    calculated by using the difference between the two orbital
    energies.
  • Period: to

    Schrödinger

    In 1926, Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. This model can be portrayed as a nucleus surrounded by an electron cloud. Where the cloud is most dense, the probability of finding the electron is greatest, and conversely, the electron is less likely to be in a less dense area of the cloud. Thus, this model introduced the concept of sub-energy levels.
  • Period: to

    Chadwick

    In 1932, James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron. With the discovery of the neutron, an adequate model of the atom became available to chemists.