At a historic meeting of the British Association for the Advancement of
Science held in Birmingham, England, in 1913, two apparently unrelated
lines of investigation were reported, each of which showed that some
atomic nuclei have identical electric charges but different weights.
One report was presented by Frederick Soddy, who had collaborated with
Rutherford in explaining the pattern of natural radioactivity. Soddy
knew that the nucleus of a radioactive atom loses both weight and
positive charge when it throws out an alpha particle (helium nucleus).
On the other hand, when a nucleus emits a beta particle (negative
electron), its positive charge increases, but its weight is practically
unchanged. Thus Soddy could deduce the weights and nuclear charges of
many radioactive products. In several cases the products of two
different kinds of radioactivity had the same nuclear charge but
different weights. Since it is the positive charge carried by the
nucleus of an atom which fixes the number of negative electrons needed
to complete the atom, the nuclear charge is really responsible for the
exterior appearance, or chemical properties, of the atom.
This conclusion was confirmed by unsuccessful efforts to separate by
chemical means different radioactive products having the same nuclear
charge but different weights. The products might have had quite
different rates of radioactive disintegration, but they appeared to
consist of chemically identical atoms of the same chemical element and
hence to belong at the _same place_ on the periodic chart of the
elements. Soddy suggested that such atoms be called _isotopes_, from a
Greek word meaning “same place.”
At the same meeting, Francis W. Aston, an assistant of Thomson,
described what happened when charged atoms, or ions, of neon gas were
accelerated in a discharge tube similar to the cathode-ray tube in which
Thomson had discovered the electron. The rapidly moving neon ions were
deflected by a magnet. Since light objects are more easily deflected
than heavy objects, the amount of deflection indicated the weight. By
making a comparison with a familiar gas like oxygen, Thomson and Aston
were actually able to measure the atomic weight of neon. To their
surprise they found two kinds of neon. About nine-tenths of the neon
atoms had an atomic weight of 20, and the remainder an atomic weight of
22.
What Thomson and Aston had done was to show that the stable element neon
is a mixture of two isotopes. A device that can do what their apparatus
did is called a mass spectrograph. (See Figure 3.) Since their time,
instruments of this type have shown that more than three-fourths of the
stable chemical elements are mixtures of two or more stable isotopes; in
fact, there are about 300 such isotopes in all. The number of known
unstable radioactive isotopes (radioisotopes), natural or man-made, is
greater than 1000 and is still growing!
[Illustration: Figure 3 _Mass spectrograph as used by Thomson and
Aston to measure the atomic weight of neon._]
NEON 20
NEON 22