Was there a connection between these two sets of particles—atoms and
electrons? In 1897, when the electron was discovered, a line of research
that was to tie the two kinds of particles together had already begun.
In 1895 the German physicist Wilhelm Konrad Roentgen (1845-1923) was
working with cathode rays. He found that if he made the cathode rays
strike the glass at the other end of the tube, a kind of radiation was
produced. This radiation was capable of penetrating glass and other
matter. Roentgen had no idea as to the nature of the radiation, and so
called it “X rays”. This name, containing “X” for “unknown”, was
retained even after physicists worked out the nature of X rays and found
them to be light-like radiation made up of waves much shorter than those
of ordinary light.
[Illustration: _Antoine Henri Becquerel._]
At once, physicists became fascinated with X rays and began searching
for them everywhere. One of those involved in the search was the French
physicist Antoine Henri Becquerel (1852-1908). A certain compound,
potassium uranyl sulfate, glowed after being exposed to sunlight and
Becquerel wondered if this glow, like the glow on the glass in
Roentgen’s X-ray tube, contained X rays.
[Illustration: Roentgen’s laboratory]
[Illustration: _Wilhelm Roentgen and his laboratory at the University of
Würzburg._]
It did, but while investigating the problem in 1896, Becquerel found
that the compound was giving off invisible penetrating X-ray-like
radiation continually, whether it was exposed to sunlight or not. The
radiation was detected because it would fog a photographic plate just as
light would. What’s more, the radiation would fog the plate, even if the
plate were wrapped in black paper, so that it could penetrate matter
just as X rays could.
Others, in addition to Becquerel, were soon investigating the new
phenomenon. In 1898 the Polish (later French) physicist Marie Sklodowska
Curie (1867-1934) showed that it was the uranium atom that was the
source of the radiation, and that any compound containing the uranium
atom would give off these penetrating rays.
Until then, uranium had not been of much interest to chemists. It was a
comparatively rare metal that was first discovered in 1789 by the German
chemist Martin Heinrich Klaproth (1743-1817). It had no particular uses
and remained an obscure element. As chemists learned to work out the
atomic weights of the various elements, they found, however, that, of
the elements then known, uranium had the highest atomic weight of
all—238.
Once uranium was discovered to be an endless source of radiation, it
gained interest that has risen ever since. Madame Curie gave the name
“radioactivity” to this phenomenon of continuously giving off rays.
Uranium was the first element found to be radioactive.
It did not remain alone, however. It was soon shown that thorium was
also radioactive. Thorium, which had been discovered in 1829 by
Berzelius, was made up of atoms that were the second most massive known
at the time. Thorium’s atomic weight is 232.
But what was the mysterious radiation emitted by uranium and thorium?
Almost at once it was learned that whatever the radiation was, it was
not uniform in properties. In 1899 Becquerel (and others) showed that,
in the presence of a magnet, some of the radiation swerved in a
particular direction. Later it was found that a portion of it swerved in
the opposite direction. Still another part didn’t swerve at all but
moved on in a straight line.
The conclusion was that uranium and thorium gave off three kinds of
radiation. One carried a positive charge of electricity, one a negative
charge, and one no charge at all. The New Zealand-born physicist Ernest
Rutherford (1871-1937) called the first two kinds of radiation “alpha
rays” and “beta rays”, after the first two letters of the Greek
alphabet. The third was soon called “gamma rays” after the third letter.
[Illustration: _Ernest Rutherford_]
[Illustration: _Marie Curie and her two daughters, Eve (left) and Irene,
in 1908._]
[Illustration: _Pierre Curie during a class lecture in 1906, the year of
his death._]
The gamma rays eventually turned out to be another light-like form of
radiation, with waves even shorter than those of X rays. The alpha rays
and beta rays, which carried electric charges, seemed to be streams of
charged particles (“alpha particles” and “beta particles”) just as the
cathode rays had turned out to be.
In 1900, indeed, Becquerel studied the beta particles and found them to
be identical in mass and charge with electrons. They _were_ electrons.
By 1906 Rutherford had worked out the nature of the alpha particles.
They carried a positive electric charge that was twice as great as the
electron’s negative charge. If an electron carried a charge that could
be symbolized as -, then the charge of the alpha particle was ++.
Furthermore, the alpha particle was much more massive than the electron.
It was, indeed, as massive as a helium atom (the second lightest known
atom) and four times as massive as a hydrogen atom. Nevertheless, the
alpha particle can penetrate matter in a way in which atoms cannot, so
that it seems much smaller in diameter than atoms are. The alpha
particle, despite its mass, is another subatomic particle.
Here, then, is the meeting point of electrons and of atoms—the particles
of electricity and of matter.
Ever since Dalton had first advanced the atomic theory over a century
earlier, chemists had assumed that atoms were the fundamental units of
matter. They had assumed atoms were as small as anything could be and
that they could not possibly be broken up into anything smaller. The
discovery of the electron, however, had shown that some particles, at
least, might be far smaller than any atom. Then, the investigations into
radioactivity had shown that atoms of uranium and thorium spontaneously
broke up into smaller particles, including electrons and alpha
particles.
It would seem, then, that atoms of these elements and, presumably, of
all elements, were made up of still smaller particles and that among
these particles were electrons. The atom had a structure and physicists
became interested in discovering exactly what that structure was.