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].)
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.