http://www.ck12.org Chapter 4. Atomic Structure
in Physics in 1903 for their work. Marie Curie later won a Nobel Prize in Chemistry (1911) for her contributions to
our understanding of radioactivity. We will study radioactivity further in the chapterNuclear Chemistry.
Discovery of the Nucleus
FIGURE 4.8
Ernest Rutherford (1871-1937).By 1900, it was known that the electron carried a negative charge. It was also known that the electron makes up
an extremely small fraction of the mass of an atom. Ernest Rutherford set out to determine how the remainder of
the mass and charge was distributed in the atom. Rutherford was a physicist from New Zealand who was working
under the direction of J. J. Thomson. He conducted several experiments on the radioactive properties of uranium. He
discovered that uranium released two different types of particles, which he referred to as alpha (α) particles, which
were positively charged, and beta (β) particles, which were negatively charged. It was later shown that beta particles
were simply free electrons.
Gold Foil Experiment
One of Rutherford’s famous experiments was called thegold foil experiment(illustrated in theFigure4.9). In
this experiment, Rutherford used a radioactive source to direct alpha particles toward a very thin sheet of gold foil.
Surrounding the foil was a screen that fluoresced when struck by the alpha particles.
Here is a short video of his experiment: http://www.youtube.com/watch?v=5pZj0u_XMbc (0:47).
MEDIA
Click image to the left for more content.As shown in the video, most alpha particles easily passed through the gold foil and struck the fluorescent screen
behind the foil. However, there were some instances in which the alpha particles were deflected very strongly,
often back toward the emission source. If the plum pudding model were correct, all of the alpha particles would
be expected to pass through the gold foil with little or no deflection. The strong deflection experienced by a small