http://www.ck12.org Chapter 24. Atomic Physics
24.1 Modeling the Atom
Objective
The student will:
- Understand Rutherford’s model of the atom.
Vocabulary
- Rutherford scattering
Introduction
The structure of the atom was first probed at the end of the 19th century, through study of radiation and radioactivity.
Three of the first forms of radiation studied were:
- X-rays, discovered in 1895 by Wilhelm C. Roentgen (1845-1923).
- Radiation from uranium, discovered in 1896 by Henri Becquerel (1852-1908) and further studied by Marie
Curie (1867-1934). - Electrons – then called “cathode rays” – were discovered decades earlier by German physicist Johann Hittorf,
but their mass was measured in 1897 by British physicist J. J. Thomson (1856-1940), who is credited with
their discovery.
Soon after, British physicist Ernest Rutherford (1871-1937) and French physicist Paul Villard separated radiation
into three types, which would eventually be named alpha, beta, and gamma. Radiation would prove to be a key
probe of the inside of the atom.
The Gold Foil Experiment
Rutherford conceived an experiment to probe the structure of the atom with the help of Hans Geiger (1882-1945),
who co-invented the Geiger counter. The actual experiment was carried out by a 19-year-old undergraduate student,
Ernest Marsden, (1889-1970) and Geiger himself.
The experiment was set up in an evacuated chamber watching alpha particles directed at an extremely thin metal
foil, using a fluorescent screen as a detector, as shown inFigure24.2. A deflected alpha particle would make a tiny
flash on the screen visible through a microscope.
Marsden’s job was to sit in the dark and look for these tiny flashes on the microscope at a spot on the screen, and
record how many flashes he saw per minute for different angles and different types of foil.
The general results were:
- Most of the particles, as expected, traveled straight through the foil. It was known that radiation would
penetrate through solid matter. - Some particles did scatter, and some even scattered back by more than 90 degrees. In these cases, the alpha
particles were repelled backward!