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Chapter 5: The Nucleus of the Atom
Aimee Gillespie, July 2006
This chapter is about Ernest Rutherford and his famous foil experiment, the results of which gave the world a new “nuclear” model of the atom. The model that was used prior to Rutherford’s model was the so-called “plum pudding model” of the atom. Joseph John (J.J.) Thomsom, the discoverer of the electron
(then called a “corpuscle”), and others proposed a model in which most of the mass of the atom was comprised of a sphere of positive charge (the pudding), and embedded within the sphere were small negatively charged corpuscles
(plums). Rutherford’s model was radically different because he said that most of the atom was empty space.
Rutherford is described as a “bulldog of a man.” He had a loud, booming voice and a gruff appearance. He was widely considered the greatest experimental physicist of his day. His manual dexterity and his physical intuition allowed him to build most of his lab equipment by hand. This skill can be attributed to his father, who was a farmer and technologist but also worked as a repairman. New Zealand is where Rutherford spent his childhood. He was a bright student and did well in all subjects. He came to Europe in 1895 when he was twenty-four years old. He held a short term research position at Cavendish
Laboratory at Cambridge University. The atmosphere there was not right for
Rutherford; he found it a bit too snobbish. He accepted a position at McGill
University, where he studied radioactivity. It was for this work that Rutherford was awarded the Nobel Prize in chemistry in 1908.
Rutherford became chair of physics at the University of Manchester in
1907 when the current holder stepped down to give Rutherford the position. It was here that he did his most important work on the structure of the atom.
Although little was known about the inside of the atom, the approximate mass and size of atoms were known from previous experiments by Lorenz and others.
Thomson knew about the charged subatomic particles the “corpuscle,” and he also deduced that there must be an equal amount of opposite charge because most atoms are electrically neutral. It was using this information that he developed the plum pudding model of the atom.
Rutherford became very intrigued by a particle that was emitted by radioactive elements, the alpha particle. The alpha particle was as massive as four hydrogen atoms and had the opposite charge of two corpuscles. He thought this was the perfect tool with which to investigate the insides of atoms.
Rutherford devised an experiment in which radioactive material sent alpha particles out of a narrow container, such that the particles formed a stream that passed through a thin foil and hit a scintillation counter, which was a zinc sulfide screen that sparked when it was struck by an alpha particle.
The plum pudding model said that the alphas should pass through the foil with little deflection because they are quite massive and traveling at a high speed. Geiger performed this experiment and his results matched what was

expected. Even so, Rutherford gave the task of measuring large deflections to
Marsden, a student who had asked him for an apprenticeship. Rutherford’s intuition was correct: Marsden and Geiger did observe large deflections in 1 out of every 20,000 alpha particle that was ejected. Rutherford is quoted as saying,
“It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you.”
This 180 degree deflection had to be caused by something that was heavy and dense in the atom, “…a peach pit in the pudding. Or perhaps a peach pit and no pudding at all.” Rutherford deduced that most of the atom is empty space and that most of the mass was concentrated at the center, in a comparatively small nucleus. A comparison that the author uses makes the relative sizes more familiar to us: “If the atom were the size of Fenway Park in Boston, then the nucleus at its center, containing all the positive charge and more than 99.9 percent of the mass, would be the size of a pea.”
Rutherford describes his findings using plenty of relatively simple calculations. He uses physics such as the conservation of momentum and energy of the alpha particle. His genius is not the complicated physics he presents, but rather the intuition that led him to investigate large deflections which were inconsistent with the present model. His new model was ignored by many, but Niels Bohr used Rutherford’s results in his quantum model of the atom.