changes did, and the percentage loss in mass was correspondingly greater. The loss of mass in radioactive changes was found to match the production of energy in just the way Einstein predicted.

It was no longer quite accurate to talk about the conservation of mass after 1905 (even though mass was just about conserved in ordinary chemical reactions so that the law could continue to be used by chemists without trouble). Instead, it is more proper to speak of the conservation of energy, and to remember that mass was one form of energy and a very concentrated form.

The mass-energy equivalence fully explained why the atom should contain so great a store of energy. Indeed, the surprise was that radioactive changes gave off as little energy as they did. When a uranium atom broke down through a series of steps to a lead atom, it produced a million times as much energy as that same atom would release if it were involved in even the most violent of chemical changes. Nevertheless, that enormous energy change in the radioactive breakdown represented only about one-half of 1% of the total energy to which the mass of the uranium atom was equivalent.

Once Rutherford worked out the nuclear theory of the atom, it became clear from the mass-energy equivalence that the source of the energy of radioactivity was likely to be in the atomic nucleus where almost all the mass of the atom was to be found.

The attention of physicists therefore turned to the nucleus.

THE STRUCTURE OF THE NUCLEUS

The Proton

As early as 1886 Eugen Goldstein, who was working with cathode rays, also studied rays that moved in the opposite direction. Since the cathode rays (electrons) were negatively charged, rays moving in the opposite direction would have to be positively charged. In 1907 J. J. Thomson called them “positive rays”.

Once Rutherford worked out the nuclear structure of the atom, it seemed clear that the positive rays were atomic nuclei from which a number of electrons had been knocked away. These nuclei came in different sizes.

Were the nuclei single particles—a different one for every isotope of every element? Or were they all built up out of numbers of still smaller particles of a very limited number of varieties? Might it be that the nuclei owed their positive electrical charge to the fact that they contained particles just like the electron, but ones that carried a positive charge rather than a negative one?

All attempts to discover this “positive electron” in the nuclei failed, however. The smallest nucleus found was that produced by knocking the single electron off a hydrogen atom in one way or another. This hydrogen nucleus had a single positive charge, one that was exactly equal in size to the negative charge on the electron. The hydrogen nucleus, however, was much more massive than an electron. The hydrogen nucleus with its single positive charge was approximately 1837 times as massive as the electron with its single negative charge.

Was it possible to knock the positive charge loose from the mass of the hydrogen nucleus? Nothing physicists did could manage to do that. In 1914 Rutherford decided the attempt should be given up. He suggested that the hydrogen nucleus, for all its high mass, should be considered the unit of positive electrical charge, just as the electron was the unit of negative electrical charge. He called the hydrogen nucleus a “proton” from the Greek word for “first” because it was the nucleus of the first element.

One proton balances 1837 electrons.

Why the proton should be so much more massive than the electron is still one of the unanswered mysteries of physics.

The Proton-Electron Theory

What about the nuclei of elements other than hydrogen?

All the other elements had nuclei more massive than that of hydrogen and the natural first guess was that these were made up of some appropriate number of protons closely packed together. The helium nucleus, which had a mass four times as great as that of hydrogen, might be made up of 4 protons; the oxygen nucleus with a mass number of 16 might be made up of 16 protons and so on.

This guess, however, ran into immediate difficulties. A helium nucleus might have a mass number of 4 but it had an electric charge of +2. If it were made up of 4 protons, it ought to have an electric charge of +4. In the same way, an oxygen nucleus made up of 16 protons ought to have a charge of +16, but in actual fact it had one of +8.

Could it be that something was cancelling part of the positive electric charge? The only thing that could do so would be a negative electric charge[1] and these were to be found only on electrons as far as anyone knew in 1914. It seemed reasonable, then, to suppose that a nucleus would contain about half as many electrons in addition to the protons. The electrons were so light, they wouldn’t affect the mass much, and they would succeed in cancelling some of the positive charge.

Thus, according to this early theory, now known to be incorrect, the helium nucleus contained not only 4 protons, but 2 electrons in addition. The helium nucleus would then have a mass number of 4 and an electric charge (atomic number) of 4 - 2, or 2. This was in accordance with observation.

This “proton-electron theory” of nuclear structure accounted for isotopes very nicely. While oxygen-16 had a nucleus made up of 16 protons and 8 electrons, oxygen-17 had one of 17 protons and 9 electrons, and oxygen-18 had one of 18 protons and 10 electrons. The mass numbers were 16, 17, and 18, respectively, but the atomic number was 16 - 8, 17 - 9, and 18 - 10, or 8 in each case.

Again, uranium-238 has a nucleus built up, according to this theory, of 238 protons and 146 electrons, while uranium-235 has one built up of 235 protons and 143 electrons. In these cases the atomic number is, respectively, 238 - 146 and 235 - 143, or 92 in each case. The nucleus of the 2 isotopes is, however, of different structure and it is not surprising therefore that the radioactive properties of the two—properties that involve the nucleus—should be different and that the half-life of uranium-238 should be six times as long as that of uranium-235.

The presence of electrons in the nucleus not only explained the existence of isotopes, but seemed justified by two further considerations.

First, it is well known that similar charges repel each other and that the repulsion is stronger the closer together the similar charges are forced. Dozens of positively charged particles squeezed into the tiny volume of an atomic nucleus couldn’t possibly remain together for more than a tiny fraction of a second. Electrical repulsion would send them flying apart at once.

On the other hand, opposite charges attract, and a proton and an electron would attract each other as strongly as 2 protons (or 2 electrons) would repel each other. It was thought possible that the presence of electrons in a collection of protons might somehow limit the repulsive force and stabilize the nucleus.

Second, there are radioactive decays in which beta particles are sent flying out of the atom. From the energy involved they could come only out of the nucleus. Since beta particles are electrons and since they come from the nucleus, it seemed to follow that there must be electrons within the nucleus to begin with.

The proton-electron theory of nuclear structure also seemed to account