• 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

• 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

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.

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.

• 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

• 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

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.