c/A simplified version of Ruther-
ford’s 1908 experiment, showing
that alpha particles were doubly
ionized helium atoms.d/These pellets of uranium
fuel will be inserted into the metal
fuel rod and used in a nuclear
reactor. The pellets emit alpha
and beta radiation, which the
gloves are thick enough to stop.nately necessary for physics students to memorize all three names
for the same thing.
At first, it seemed that neither alphas or gammas could be de-
flected in electric or magnetic fields, making it appear that neither
was electrically charged. But soon Rutherford obtained a much more
powerful magnet, and was able to use it to deflect the alphas but
not the gammas. The alphas had a much larger value ofm/qthan
the betas (about 4000 times greater), which was why they had been
so hard to deflect. Gammas are uncharged, and were later found to
be a form of light.
Them/q ratio of alpha particles turned out to be the same
as those of two different types of ions, He++(a helium atom with
two missing electrons) and H+ 2 (two hydrogen atoms bonded into a
molecule, with one electron missing), so it seemed likely that they
were one or the other of those. The diagram shows a simplified ver-
sion of Rutherford’s ingenious experiment proving that they were
He++ions. The gaseous element radon, an alpha emitter, was in-
troduced into one half of a double glass chamber. The glass wall
dividing the chamber was made extremely thin, so that some of the
rapidly moving alpha particles were able to penetrate it. The other
chamber, which was initially evacuated, gradually began to accu-
mulate a population of alpha particles (which would quickly pick up
electrons from their surroundings and become electrically neutral).
Rutherford then determined that it was helium gas that had ap-
peared in the second chamber. Thus alpha particles were proved to
be He++ions. The nucleus was yet to be discovered, but in modern
terms, we would describe a He++ion as the nucleus of a He atom.
To summarize, here are the three types of radiation emitted by
radioactive elements, and their descriptions in modern terms:
αparticle stopped by a few inches of air He nucleus
βparticle stopped by a piece of paper electron
γray penetrates thick shielding a type of light
Discussion Question
A Most sources of radioactivity emit alphas, betas, and gammas, not
just one of the three. In the radon experiment, how did Rutherford know
that he was studying the alphas?
8.2.2 The planetary model
The stage was now set for the unexpected discovery that the
positively charged part of the atom was a tiny, dense lump at the
atom’s center rather than the “cookie dough” of the raisin cookie
model. By 1909, Rutherford was an established professor, and had
students working under him. For a raw undergraduate named Mars-
den, he picked a research project he thought would be tedious but
straightforward.Section 8.2 The nucleus 497