• 384

    Aristotle (348 BC)

    Aristotle (348 BC)
    Aristotle said that the atom was based on four separate elements, fire, water, earth, and air. Also, there were four qualities to the elements -dryness, hotness, coldness, and moistness. This view was held for 2,000 years, until disproved.
  • 460

    Democritus (460-370 BC)

    Democritus (460-370 BC)
    Atoms are present in the atmosphere. They are present in all forms (liquid, gas, solid) He also thought that atoms are all individually created and cannot be separated under any sort of scientific circumstance. Atoms cannot be split. no matter what their size. Adopted in 460 BC. he used no experiments to conclude his theory, just figured matter could not be divided up past a certain point
  • John Dalton (1803)

    John Dalton (1803)
    His theory was that all matter is made of indivisible and indestructible atoms. Atoms cannot be made, or destroyed. So because of this, all atoms of a given element are identical in mass and properties, and compounds are formed by a combination of 2 or more different types of atoms. also, he said a chemical reaction is simply just a rearrangement of atoms. He was inspired by Democritus and Lavoiser.
  • Antoine-Henri Becqurerel (1896)

    Antoine-Henri Becqurerel (1896)
    His radiation experiments showed the instability of the atomic nuclei. His many radioactive experiments for radiation began to show the theory of the atomic structure, he stumbled upon the atomic structure per accident. Helped by Marie&Pierre Curie.
  • Marie&Pierre Curie (1898)

    Marie&Pierre Curie (1898)
    Made the understanding of radioactivity more thorough through their research of radioactive materials. They discovered radium and polonium. Shown what atoms were like when affected. Worked with Henri Becquerel.
  • J.J. Thomson (1898

    J.J. Thomson (1898
    Used an experiment with a cathode ray tube, that discovered electrons. Using the "Plum Pudding" model, it added negatively charged electrons to the full atom. This maintained charge neutrality.
  • Robert Millikan (1908)

    Robert Millikan (1908)
    He discovered the charge of the electron. Discovered this with his "Oil Drop" Experiment. This proved the mass of an electron is 1000 times smaller than the smallest atom. Harvey Fletcher helped him with this experiment.
  • Ernest Rutherford (1909)

    Ernest Rutherford (1909)
    He said that all of the positive charge and all of the mass of the atom occupied a small volume at the center of an atom which shows that most of the volume of the atom was empty and full of the electrons. Used the "Gold-Foil" Experiment(shooting particles at gold foil expecting them to go through but bounced back) to prove this. Proving the existence of the nucleus. Tried to propose the existense of the neutron. Tried proving J.J. Thomson's model.
  • Niels Bohr (1913)

    Niels Bohr (1913)
    He applied his theory to "Rutherford's Atomic Structure" and just assumed that electrons traveled in orbits determined by the energy levels. Consulted with Ernest Rutherford,
  • Quantum Model (1923)

    Quantum Model (1923)
    Approximately where the electron is located. Most up to date theory on atomic structure. Explains the stability of the electrons surrounding a nucleus. Includes nucleus(center of atom) electon (negative charge) neutron (no charge) and the proton (positive charge) in the atom. All past experiments and theories have proved or lead up to the Quantum Theory.
  • James Chadwick (1932)

    James Chadwick (1932)
    Proved their was a neutral force within the atom. Idea of neutron proposed earlier by Rutherford. Used sample Berrylium with alpha-particles, which emits radiation (electrons and protons would cancel eachother out)