JJ Thomson discovered the electron through his work with cathode ray tubes in the late 1800s. This led him to propose the plum pudding model of the atom, with negatively charged electrons embedded in a positively charged medium. Robert Millikan later directly measured the charge of individual electrons through his oil drop experiment in 1905, enabling him to calculate the electron's mass. Ernest Rutherford then used the gold foil experiment to discover the nucleus as the small, dense, positively charged center of atoms through deflections of alpha particles in 1910.

1. JJ. Thomson (1856-1940)
English Physicist
Discovered the Electron
- Studied the Cathode Ray Tube
- Found the mass to charge ratio of an electron
- Led to the Plum Pudding Model

2. The Design of the Cathode Ray Tube

3. So! Thompson has
discovered the
electron, and it must
live inside atoms.
It is much less massive
than the the atom itself,
so perhaps we have
little electrons stuffed
into the ‘rest’ of the
atom like raisins in the
oatmeal, or:
Plum Pudding...

4. Robert Milliken (1868-1953)
American Physicist
Performed the Oil Drop
Experiment
– Measured minimum electric charge that
could be carried by a particle (1905)
– This was the charge of the electron
– Enabled him to find the mass of the
electron

5. Millikan’s Oil Drop Experiment
Used To Determine the Charge of an Electron

6. Ernest Rutherford (1871-1953)
• New Zealand born physicist
• Worked with nuclear radiation
• Performed the Gold Foil Experiment
• Discovered the nucleus (1910)
• Discovered the proton (1918)

7. The Gold Foil Experiment
Demonstrated that the atom contains a nucleus
which is:
a. Very tiny – most of the particles missed
b. Positively charged – some were deflected
c. Very dense – some bounced off

8. James Chadwick (1891-1974)
• Discovered the neutron in 1932
• In college he stood in the wrong line.
He wanted to be a mathematician, but
was in the line for physics and was too
embarrassed to switch.
• Instead he became a famous physicist.

9. End!
The story’s not over, but we’ll pick it up in
chapter 4.