1. The Proton, Neutron and Electron
Subatomic Particles
The Proton, Neutron and Electron

2. The Electron The negatively charged subatomic particle.
Discovered by J.J. Thomson using a cathode ray tube. (vacuum tube)
Symbol = e-
Charge* = -1 ( X C)
Relative Mass* = 0
Mass in grams = X g

3. particles (electrons)
A Cathode Ray Tube
Source of
Electrical
Potential
Metal Plate
Gas-filled
glass tube
Metal plate
Stream of negative
particles (electrons)
J. J. Thomson - English physicist. 1897
Made a piece of equipment called a cathode ray tube.
It is a vacuum tube - all the air has been pumped out.
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

4. Background Information
Cathode Rays
Form when high voltage is applied across electrodes in a partially evacuated tube.
Originate at the cathode (negative electrode) and move to the anode (positive electrode)
Travel in straight lines in the absence of an external field

5. A Cathode Ray Tube
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

6. Results of Cathode Ray Experiment
Using a cathode ray tube, Thomson was able to deflect cathode rays with an electrical field.
The rays bent towards the positive pole, indicating that they are negatively charged.

7. The Effect of an Electric Field on Cathode Rays
High
voltage
cathode
source of
high voltage
positive
plate
negative
anode _ +
Charged particles tend to move away from particles with the same charge and toward particles with the opposite charge. When the cathode rays bent away from the negative pole of the magnet and toward the positive pole, this rule caused Thomson to realize the cathode rays were negatively charged.
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117

8. - Thomson’s Experiment + voltage source vacuum tube metal disks
J. J. Thomson - English physicist. 1897
Made a piece of equipment called a cathode ray tube.
It is a vacuum tube - all the air has been pumped out.
vacuum tube
metal disks

9. - Thomson’s Experiment + voltage source
OFF +
Passing an electric current makes a beam appear
to move from the negative to the positive end

10. - Thomson’s Experiment + voltage source + -
OFF + + -
By adding an electric field…
he found that the moving pieces were negative.

11. Conclusions
He compared the value with the mass/ charge ratio for the lightest charged particle.
By comparison, Thomson estimated that the cathode ray particle weighed 1/1000 as much as hydrogen, the lightest atom.
He concluded that atoms do contain subatomic particles - atoms are divisible into smaller particles.
This conclusion contradicted Dalton’s postulate and was not widely accepted by fellow physicists and chemists of his day.
In the cathode ray experiment, Thomson was able to verify the existence of the electron, characterize its mass, and determine that it has negative charge.

12. Thomson’s Model of the Atom
Plum Pudding model
Negative electrons were embedded into a positively charged spherical cloud.
Spherical cloud of
Positive charge
Electrons
Named after a dessert, the plum pudding model portrays the atom as a big ball of positive charge containing small particles with negative charge.
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56

13. The Proton
Discovered by Ernest Rutherford during his Gold Foil Experiment.
Positively charged subatomic particle.
Symbol = p+
Charge = +1 (equal but opposite electron)
( X C)
Relative mass = 1*
Actual mass = X g

14. The Neutron Subatomic particle with no charge (neutral)
Discovered by James Chadwick (1932)
Symbol = n0
Charge = 0
Relative Mass = 1*
Actual Mass = X g