deflected through large angles, and a very few -particles even returned from the gold
foil in the direction from which they had come! Rutherford was astounded. In his own
words,It was quite the most incredible event that has ever happened to me in my life. It was almost
as if you fired a 15-inch shell into a piece of tissue paper and it came back and hit you.Rutherford’s mathematical analysis of his results showed that the scattering of positively
charged -particles was caused by repulsion from very dense regions of positive charge in
the gold foil. He concluded that the mass of one of these regions is nearly equal to that
of a gold atom, but that the diameter is no more than 1/10,000 that of an atom. Many
experiments with foils of different metals yielded similar results. Realizing that these obser-
vations were inconsistent with previous theories about atomic structure, Rutherford
discarded the old theory and proposed a better one. He suggested that each atom contains
a tiny, positively charged, massive centerthat he called an atomic nucleus.Most -particles
pass through metal foils undeflected because atoms are primarilyempty space populated
only by the very light electrons. The few particles that are deflected are the ones that
come close to the heavy, highly charged metal nuclei (Figure 5-5).
Rutherford was able to determine the magnitudes of the positive charges on the atomic
nuclei. The picture of atomic structure that he developed is called the Rutherford model
of the atom.Atoms consist of very small, very dense positively charged nuclei surrounded by
clouds of electrons at relatively great distances from the nuclei.Ernest Rutherford was one of the
giants in the development of our
understanding of atomic structure.
While working with J. J. Thomson
at Cambridge University, he
discovered and radiation. He
spent the years 1899–1907 at McGill
University in Canada where he
proved the nature of these two
radiations, for which he received the
Nobel Prize in chemistry in 1908.
He returned to England in 1908,
and it was there, at Manchester
University, that he and his coworkers
Hans Geiger and Ernst Marsden
performed the famous gold foil
experiments that led to the nuclear
model of the atom. Not only did he
perform much important research in
physics and chemistry, but he also
guided the work of ten future
recipients of the Nobel Prize.
182 CHAPTER 5: The Structure of Atoms
Figure 5-5 An interpretation of the Rutherford scattering experiment. The atom is
pictured as consisting mostly of “open” space. At the center is a tiny and extremely dense
nucleus that contains all of the atom’s positive charge and nearly all of its mass. The
electrons are thinly distributed throughout the “open” space. Most of the positively charged
-particles (black arrows) pass through the open space undeflected, not coming near any gold
nuclei. The few that pass fairly close to a nucleus (red arrows) are repelled by electrostatic
forces and thereby deflected. The very few particles that are on a “collision course” with
gold nuclei are repelled backward at acute angles (blue arrows). Calculations based on the
results of the experiment indicated that the diameter of the open-space portion of the atom
is from 10,000 to 100,000 times greater than the diameter of the nucleus.This representation is notto scale. If
nuclei were as large as the black dots
that represent them, each white region,
which represents the size of an atom,
would have a diameter of more than
30 feet!