The Structure of the Atom 165
exposed. Apparently, potassium uranyl sulfate emits radiation on its own
without being exposed to the sunlight. Becquerel isolated the source of
the spontaneous of radiation and found that it was due to uranium.
Becquerel’s effort led to the work of Pierre and Marie Curie who went
on to investigate the nature of radioactivity (the spontaneous emission of
radiation from matter, a term coined by Marie Curie) and to isolate a
number of radioactive elements such as polonium, thorium and radium.
Subsequent investigation revealed that radioactive atoms emitted
three forms of radiation, which were labeled alpha rays, beta rays and
gamma rays. Beta rays were identified by Becquerel as high-energy
electrons. The gamma ray was discovered by P. Villard and identified
as electromagnetic radiation with a wavelength even shorter than that of
x-rays. Alpha rays were discovered by the Curies and Earnest Rutherford
and were identified as a stream of particles with the charge, +2e, and a
mass four times that of the hydrogen atom. In other words, the alpha
particle is nothing more than the nucleus of the helium atom without its
two electrons. The ultimate source of radioactivity within the atom is
the nucleus and we shall, therefore, defer our discussion of radioactivity
until we come to our treatment of nuclear physics. The importance
of radioactivity for the study of atomic physics is twofold. Firstly, it
provides, in itself, additional evidence for the composite nature of the
atom. Secondly, and perhaps more importantly, Rutherford realized that,
because of its charge, alpha rays could be used as a tool to probe the
structure of the atom. He and his colleagues, Marsden and Geiger,
devised a scattering experiment in which a collimated beam of alpha
particles was directed at a thin gold foil target. A schematic sketch of
the experimental setup is shown in Fig. 17.3. The alpha rays originated
at the source and pass through a slit in a thick sheet of lead in order
to produce a collimated beam. The collimated beam then passes through
the gold foil where it is scattered by the individual gold atoms. The
scattered alpha rays are then detected with a fluorescent screen to
which is attached an eyepiece. Whenever an alpha particle strikes the
fluorescent screen, a spark of light is produced, which is visible through
the eyepiece. The fluorescent screen and eyepiece can be moved about
the circle so that the number of alpha particles scattered at each angle can
be counted.
When Rutherford and his colleagues performed their experiment,
they believed that they would establish the experimental validity of
J.J. Thomson’s plum pudding picture of the atom in which a number of