History of Atomic Theory

By 001196592
  • 482 BCE

    Particles

    2,500 years ago, Greek Philosopher Leucippus and his pupil Democritus first thought that matter is composed of tiny particles. They gave these particles the name of "atomos", which means indivisible. They also believe that every substance was made up of its own particles, as in cheese was made of small cheese particles.

  • Law of Conservation of Mass

    Antoine Lavoisier came around about 2,300 years later and proposed the Law of Conservation of Mass stating that even if matter changes shape or form its mass stays the same.

  • Dalton's Theory

    In 1803, John Dalton had a theory stating that elements exist as discreet packets of matter.

  • Discharge Tubes

    During the 1870's, scientists began to use discharge tubes to probe what things were composed of.

  • Positive Electrodes in Cathode Rays

    In 1886, Eugen Goldstein found that the tubes could also emit light from the positive electrode. Because it was originally produced by a negative electrode, but it also emitted light from the positive electrode this told him that there must also be a positive charge in matter.

  • Electron Discovery

    In 1897, Joseph John Thomson discovered that cathode rays are very small, very light particles. We know them today as electrons, but he called them corpuscles.

  • Nucleus Discovery

    In 1909, Earnest Rutherford conducted an experiment which brought him to the conclusion that the entire positive charge in an atom must be concentrated in a very small area. He named this area the nucleus.

  • Proton Discovery

    In 1910, Ernest Rutherford also discovered that if you bond alpha and nitrogen particles, you can create hydrogen ions. He stated that these ions were fundamental particles, also known as protons.

  • The Bohr Model

    In 1911, Niels Henrik David Bohr made a model of an atom that can be used to make a somehwat accurate model of any element.

  • Quantom Theory

    In 1925, Werner Karl Heisenberg discovered that it is impossible to know the exact momentum or exact potential of an electron, or particle. He proposed that the number of electrons in an atom aren't particles or waves, but instead had properties of both and neither at the same time. His thought of a hypothesis stating that there are regions in an atom where electrons are much more likely to be. Hee called these regions orbitals.