[Illustration: _Ernest Rutherford, 1871-1937_
Courtesy Nobelstiftelsen]
We are greatly indebted to the imagination and experimental skill of the
British physicist Ernest Rutherford for the interpretation of
radioactivity in terms of the structure of atoms.
Rutherford, born and educated in New Zealand, moved to England to work
under Thomson at Cambridge University in 1895. Shortly afterward,
Wilhelm Roentgen in Germany discovered X rays, Becquerel in France
discovered radioactivity, and Thomson proved the existence of the
electron.
During the next few years, curiosity about the fundamental nature of
radioactivity led a number of people to do a great deal of work. The
element thorium was found to be radioactive, and Marie and Pierre Curie
discovered two new elements, polonium and radium, that were also
radioactive. The radiation from radioactive materials was found to be of
three kinds called alpha rays, beta rays, and gamma rays. Alpha rays
were first detected by Rutherford, who later identified them as
positively charged helium atoms. Becquerel demonstrated that beta rays,
like cathode rays, consist of negatively charged electrons. The highly
penetrating gamma rays were proved by Rutherford and E. N. da C. Andrade
to be electromagnetic radiation similar to X rays.
Rutherford, in collaboration with the English chemist Frederick Soddy,
brought order out of a chaos of puzzling discoveries by establishing the
general behavior of radioactive atoms. He determined that certain
naturally occurring atoms of high atomic weight can spontaneously emit
an alpha or a beta particle and thereby convert themselves into new
atoms. These new atoms, being also radioactive, sooner or later convert
themselves into still different atoms, and so on. Each time an alpha
particle is emitted in this sequence, the new atom is lighter by the
weight of the alpha particle, or helium atom. The disintegration process
proceeds from stage to stage until at last a _stable_ atom is produced.
The end product in this “decay” process in naturally occurring
radioactive elements is lead.
One experiment by Rutherford and his co-workers had a most profound
effect on the understanding of atomic structure. What they did was to
direct a stream of alpha particles at a thin piece of gold foil. The
results were astonishing. Almost all the particles passed straight
through the foil without changing direction. Of the few particles that
did ricochet in new directions, however, some were deflected at very
sharp angles. (See Figure 2.)
[Illustration: Figure 2 _Rutherford’s most famous experiment, which
led him to the concept of the nucleus._]
As a result of this experiment, Rutherford proposed a concept of the
atom entirely different from the one which prevailed at this time. The
prevailing notion was one advanced by Thomson which conceived of an atom
as a blob of positive electric charge in which were imbedded, in much
the same way as plums are in a pudding, enough electrons to neutralize
the positive charge. Rutherford’s concept, which quickly set aside
Thomson’s “plum pudding” model, was that an atom has all of its positive
charge and virtually all of its mass concentrated in a tiny space at its
center. (Collisions with this center, which came to be known thereafter
as the nucleus, had been responsible for the sharp changes in direction
of some of the alpha particles.) The space surrounding this nucleus is
entirely empty except for the presence of a number of electrons (79 in
the case of the gold atom), each about the same size as the nucleus.
To illustrate Rutherford’s concept, let us imagine a gold atom magnified
so that it is as large as a bale of cotton. The nucleus at the center of
this large atom would be the size of a speck of black pepper. If this
imaginary bale weighed 500 pounds, the little speck at its center would
weigh 499¾ pounds; the surrounding cotton (corresponding to empty space
in Rutherford’s concept) containing the 79 electrons would weigh but ¼
pound. To express this idea another way, any object such as a gold ring,
as dense and solid as it may seem to us, consists almost entirely of
nothing!