1. Modern Version of the Experiment: Using electricity, water is electrolyzed into its components, hydrogen and oxygen. Results: Water is always found to contain hydrogen and oxygen in a mass ratio of 1:8 (volume ratio of 2:1) no matter how it is formed or where it is found. Scientist: Joseph Proust, 1799

2. Experiment: Wrote a book entitled: A New System of Chemical Philosophy (1808) Results: 1. All matter is composed of tiny indivisible particles called atoms. 2. Atoms of the same element are exactly the same; atoms of different elements are different. 3. Atoms combine in whole number ratios to form compounds. A given compound always has the same relative number and kind of atoms. 4. Atoms cannot be created or destroyed in chemical reactions, they are merely rearranged. Scientist: John Dalton, 1808

3. Scientist: J.J. Thomson, 1897 Experiment: An electric current is passed through an evacuated and sealed glass tube. Results: The voltage causes negative particles to move from the negative electrode to the positive electrode. The path of the electrons can be altered (deflected) by the presence of a magnetic field depending on the applied magnetic and electric fields.

4. Scientist: Robert Millikan, 1909 Experiment: Oil drops are sprayed above a positively charged plate containing a small hole. As the oil drops fall through the hole, they are given a negative charge. Gravity forces the drops downward. The applied electric field forces the drops upward. When a drop is perfectly balanced, the weight of the drop is equal to the electrostatic force of attraction between the drop and the positive plate. Results: Using this experiment, Millikan determined the charge on the electron to be 1.60  10 -19 C.

5. Experiment: A radioactive substance is placed in a shield containing a small hole so that a beam of radiation is emitted from the hole. The radiation is passed between two electrically charged plates and detected. Results: Three spots are noted on the detector: –a spot in the direction of the positive plate, –a spot which is not affected by the electric field, –a spot in the direction of the negative plate. Scientist: Henri Becquerel, 1896

6. Experiment: A source of α- particles was placed at the mouth of a circular detector. The α -particles were shot through a piece of gold foil. Results: Most of the α - particles went straight through the foil without deflection. Some α-particles were deflected at high angles. Scientist: Ernest Rutherford, 1910

7. Scientist: James Chadwick, 1932 Background: At the time, it was well known that hydrogen had 1 proton, and helium had 2 protons, but the ratio of the masses of a hydrogen atom to a helium atom is 1:4. Why? Experiment: A thin sheet of beryllium was bombarded with alpha particles, and a very high energy radiation similar to gamma rays was emitted by the metal. Results: The radiation was electrically neutral. This radiation was found to be another particle which had no charge but a slightly larger mass than a proton.