Discovery of the Electron by J. J. Thompson (Late 1890s):
View an animation of Thomson’s experiment via the link on my website
J. J. Thompson discovered the electron when he was experimenting with gas discharge tubes. He
noticed a movement in a tube. He called the movement cathode rays. The rays moved from the
negative end of the tube to the positive end. He realized that the rays were made of negatively
charged particles. During a lecture at the Royal Institute of Great Britain in 1897, J. J. Thomson
announced that he had discovered a negatively charged particle one thousand times smaller than
an atom. Thomson’s announcement was met with disbelief. This disbelief was not a shock since
Thomson himself had firmly believed for many years that atoms were indivisible. His research
was solid, and Thomson could not deny the results of his experiments. Thomson called the newly
discovered particles corpuscles, meaning “small bodies”. Today these particles are called
electrons. Based on his discovery, J. J. Thomson proposed a new model of the atom. Since atoms
were neutral, he assumed that atoms must also be composed of positively charged particles to
balance the negatively charged particles. His model showed the atom as a diffuse ball of positive
charge with negative particles dispersed
1. What did J.J. Thompson discover? _____________________________________________
3. What are cathode rays made of? ______________________________________________
4. Why do electrons move from the negative end of the tube to the positive end?
_________________________________________________________________
5. How would you describe J. J. Thomson’s model of the atom?
(a) An atom is indivisible.
(b) The atom is composed of only negatively charged particles.
(c) The atom is composed of a positive core with negatively charged particles
orbiting the positive core.
(d) The atom is composed of diffuse ball of positive charge with negative
particles dispersed throughout.
6. Draw a picture of Thomson’s Model of the atom.
Discovery of the Proton (1913)
View the two links to Rutherford’s experiment before answering the questions.
Below is a representation of Rutherford’s gold foil experiment. The paths of the alpha particles
(helium nuclei) are represented below. Use the diagram to answer the questions.
____ 1. Which path represents the path of that the
majority of the alpha particles would take?
a) 1
b) 2
c) 3
d) 4
____ 2. Which of the paths was least likely to
occur?
a) 1
b) 2
c) 3
d) 4
____ 3. The charge on the alpha particle is positive.
Based on the information the charge on the
nucleus must be…
a) positive
b) negative
c) neutral
____ 4. According to Rutherford’s experiment, the
atom is composed largely of…
a) alpha particles
b) a charged nucleus
c) empty space
d) none of the above
ANSWER IN YOUR NOTEBOOK:
5. What was the purpose of the fluorescent screen?
6. How does the path of a charged particle that strikes the center of a gold atom differ from the path of a
particle that passes near the center?
7. What would have been observed if the positive charge of an atom has been evenly spread throughout
the atom?
Discovery of the Neutron by James Chadwick (1932)
Watch the video, Atomic Structure: Discovery of the Neutron, to answer the questions:
1. What was it about the charges of protons in the nucleus of an atom made atomic scientists
hypothesize that there may be neutral subatomic particles also in the nucleus?
2. How much does one proton weight in AMU’s (atomic mass units)? ___________
3. Explain the second reason that led to the hypothesis that there must be a neutral particle in the
nucleus that regards the scientists’ knowledge of the atomic number and mass of individual atoms.
4. In James Chadwick’s experiment, what shot out of the paraffin when hit by the neutrally charged
radiation?
5. Why couldn’t the neutral radiation be gamma rays?
6. Why does the radiation shot out of the beryllium have to be about the same mass as the protons?