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History of the Atom

By Bethmorris22
  • 400 BCE

    Democritus

    Democritus
    Democritus’s found that matter consists of invisible particles called atoms and a void (empty space). Democritus called these infinitesimally small pieces of matter atomos, meaning "indivisible." Democritus hypothesized that atoms cannot be destroyed, differ in size, shape and temperature, are always moving, and are invisible. This discovery came about more by observation in normal things, not necessarily a break through from doing an experiment.

  • John Dalton

    John Dalton
    1) All matter is made of atoms. Atoms are indivisible and indestructible.
    2) All atoms of a given element are identical in mass and properties
    3) Compounds are formed by a combination of two or more different kinds of atoms.
    4) A chemical reaction is a rearrangement of atoms.
    Dalton's experiments on gases led to his discovery that the total pressure of a mixture of gases amounted to the sum of the partial pressures that each individual gas exerted while occupying the same space.

  • Joseph John Thomson

    Joseph John Thomson
    Thomson proposed that the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges.
    The first experiment was done in order to see if the charge was separate from the cathode rays. Next, J.J. Thomson set out to determine if the charge carried by the cathode rays was negative or positive. In his last experiment, He decided to try to work out the nature of the particles.

  • Ernest Rutherford

    Ernest Rutherford
    Rutherford tested Thomson's hypothesis by devising his "gold foil" experiment. Rutherford was forced to discard the Plum Pudding model and reasoned that the only way the alpha particles could be deflected backwards was if most of the mass in an atom was concentrated in a nucleus. He thus developed the planetary model of the atom which put all the protons in the nucleus and the electrons orbited around the nucleus like planets around the sun.

  • Niels Bohr

    Niels Bohr
    1) Electrons assume only certain orbits around the nucleus. These orbits are stable and called "stationary" orbits. 2) Each orbit has an energy associated with it. 3) Light is emitted when an electron jumps from a higher orbit to a lower orbit and absorbed when it jumps from a lower to higher orbit. 4)The energy and frequency of light emitted or absorbed is given by the difference between the two energies.
    He came up with this hypothesis by observing Rutherford's atomic model, not by experiment.