The Foundations of Chemistry

RUTHERFORD AND THE NUCLEAR ATOM

By the early 1900s, it was clear that each atom contains regions of both positive and nega-
tive charge. The question was, how are these charges distributed? The dominant view of
that time was summarized in J. J. Thomson’s model of the atom; the positive charge was
assumed to be distributed evenly throughout the atom. The negative charges were pictured
as being imbedded in the atom like plums in a pudding (the “plum pudding model”).
Soon after Thomson developed his model, tremendous insight into atomic structure
was provided by one of Thomson’s former students, Ernest Rutherford (1871–1937), who
was the outstanding experimental physicist of his time.
By 1909, Ernest Rutherford had established that alpha () particles are positively
charged particles. They are emitted at high kinetic energies by some radioactive atoms,
that is, atoms that disintegrate spontaneously. In 1910, Rutherford’s research group carried
out a series of experiments that had an enormous impact on the scientific world. They
bombarded a very thin piece of gold foil with -particles from a radioactive source. A
fluorescent zinc sulfide screen was placed behind the foil to indicate the scattering of the
-particles by the gold foil (Figure 5-4). Scintillations (flashes) on the screen, caused by
the individual -particles, were counted to determine the relative numbers of -particles
deflected at various angles. Alpha particles were known to be extremely dense, much denser
than gold.
If the Thomson model of the atom were correct, any -particles passing through the
foil would have been deflected by very small angles. Quite unexpectedly, nearly all of the
-particles passed through the foil with little or no deflection. A few, however, were

5-4

-Particles are now known to be He^2 

ions, that is, helium atoms without

their two electrons. (See Chapter 26.)

Radioactivity is contrary to the

Daltonian idea of the indivisibility of

atoms.

5-4 Rutherford and the Nuclear Atom 181

Figure 5-4 The Rutherford scattering experiment. A narrow beam of -particles from a
radioactive source was directed at a very thin sheet of gold foil. Most of the particles passed
right through the gold foil (brown). Many were deflected through moderate angles (shown in
red). These deflections were surprising, but the 0.001% of the total that were reflected at
acute angles (shown in blue) were totally unexpected. Similar results were observed using foils
of other metals.

Lead block

(for shielding)

Slits

Alpha source

Gold foil

Scintillation screen

Alpha beam