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o   Leucippus and Democritus came up with the idea that matter is composed of tiny particles

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o   Antoine Lavoisier proposed the law of conservation of mass

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o   James Dalton discovered that elements exist as discreet packets of matter

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o   In the 1870s, scientists used discharge tubes to investigate what things were made of

Gas-filled tube with electrodes at each end that emit light when an electrical current passes through them

Called a cathode ray since light was produced by a negative electrode

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o   In 1886, Eugen Goldstein found that tubes also emitted light from the positive electrode

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o   J.J. Thompson was able to estimate the mass of rays (1000x lighter than a hydrogen)

Measured how much heat the cathode ray generated

Concluded that rays are very light and small negatively charged particles (electrons)

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o   1909, Ernest Rutherford designed an experiment using a sheet of gold foil and a zinc sulfide-coated screen

Filled the sheet with alpha particles and found that some particles passed through the foil, while others deflected

Discovered that the positive charge of an atom must be concentrated in a very small area (the nucleus)

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o   1911, Bohr studied with Rutherford and applied mathematical principles to Rutherford’s atomic model

Allowed him to predict the most likely positions of electrons within the atom

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o   Werner Heisenberg discovered it is impossible to know both the momentum of an electron and its exact position

Proposed the quantum theory, which proposed that electrons weren’t particles or waves, and instead had properties of both and neither

By this thinking, the arrangements of electrons could only be described in terms of probability

Orbitals are regions where electrons are more likely to be found