One more piece of information must be fitted into the story of the atom
before it becomes clear why some people began to realize during the
1920s that atomic nuclei contain vast stores of energy that might some
day revolutionize civilization. This last item has to do with a nuclear
phenomenon known as the packing fraction.
Since any nucleus consists of a certain number of protons and neutrons,
it seems logical that the total weight of the nucleus could be
determined by adding together the individual weights of the particles in
it. When mass spectrographs of sufficiently high accuracy became
available, however, it was found that in the case of nuclear weights,
the whole was not equal to the sum of its parts! All nuclei (except
hydrogen) weigh less than the sum of the weights of the particles in
them.
For example, the atomic weight of a proton is 1.00812 and that of a
neutron is 1.00893. (These are relative weights based on an
internationally accepted scale.) It would seem then that a nucleus of
helium containing two protons and two neutrons should have an atomic
weight of 2 × 1.00812 plus 2 × 1.00893 or 4.0341. Actually the atomic
weight of helium as measured by the mass spectrograph is only 4.0039.
(See Figure 4.)
[Illustration: Figure 4 _A case where the whole is not equal to the
sum of its parts. Two protons and two neutrons are distinctly
heavier than a helium nucleus, which also consists of two protons
and two neutrons. Energy makes up the difference._]
HELIUM NUCLEUS
TWO PROTONS AND TWO NEUTRONS
What happens to the missing atomic weight of 0.0302? Physicists now
realize that, as postulated in Einstein’s formula, it must be converted
into energy! The conversion occurs when the protons and neutrons are
drawn together into a helium nucleus by the powerful nuclear forces
between them.
When the missing atomic weight 0.0302 is multiplied by the square of the
velocity of light according to Einstein’s theory, it is found to
represent a tremendous amount of energy. Indeed, the energy released in
forming a helium nucleus from two protons and two neutrons turns out to
be seven million times that released when a carbon atom combines with an
oxygen molecule to produce a molecule of carbon dioxide in the familiar
process of combustion.
The general behavior of such losses in atomic weight for atoms
throughout the periodic table had been determined as early as 1927,
largely through the work of Aston, the English scientist who developed
the first mass spectrograph. His results show that, in general, if two
light nuclei combine to form a heavier one, the new nucleus does not
weigh as much as the sum of the original ones. This behavior continues
up to the level of the so-called “transition metals”—iron, nickel, and
cobalt—in the periodic table. But if two nuclei heavier than iron are
coalesced into a single very heavy nucleus found near the end of the
periodic table (such as uranium), the new nucleus weighs more than the
sum of the two nuclei that formed it.
Thus, if a very heavy nucleus could be divided into parts, energy would
be released, and the sum of the weights of the fragments would be less
than that of the original nucleus.
In these two types of nuclear reactions, a small amount of matter would
actually vanish! Einstein’s Special Theory of Relativity states that the
vanished matter would reappear as an enormous quantity of energy.
During the late 1920s scientists began saying that a small amount of
matter could supply enough energy to drive a large ship across the
ocean. As we know, this prediction has since been borne out by the
performance of nuclear submarines and surface vessels.
[Illustration: _The NS_ Savannah _was the first cargo-passenger ship
to be driven by nuclear power_.
Courtesy States Marine Lines]
[Illustration: _The_ Nautilus _was the Navy’s first atomic-powered
submarine_.
Courtesy U. S. Navy]