The Story of Nuclear Energy: The Proton-Electron Theory

Worlds Within Worlds: The Story of Nuclear Energy, Volume 2 (of 3), by Isaac Asimov is part of HackerNoon’s Book Blog Post series. You can jump to any chapter in this book here. Volume II, THE STRUCTURE OF THE NUCLEUS: The Proton-Electron Theory

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 77oxygen 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 78two—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 neatly for many of the facts of radioactivity.

Why radioactivity at all, for instance? The more complex a nucleus is, the more protons must be squeezed together and the harder, it would seem, it must be to keep them together. More and more electrons seemed to be required. Finally, when the total number of protons was 84 or more, no amount of electrons seemed sufficient to stabilize the nucleus.

79

The manner of breakup fits the theory, too. Suppose a nucleus gives off an alpha particle. The alpha particle is a helium nucleus made up, by this theory, of 4 protons and 2 electrons. If a nucleus loses an alpha particle, its mass number should decline by 4 and its atomic number by 4 - 2, or 2. And, indeed, when uranium-238 (atomic number 92) gives off an alpha particle, it becomes thorium-234 (atomic number 90).

Suppose a beta particle is emitted. A beta particle is an electron and if a nucleus loses an electron, its mass number is almost unchanged. (An electron is so light that in comparison with the nucleus, we can ignore its mass.) On the other hand, a unit negative charge is gone. One of the protons in the nucleus, which had previously been masked by an electron, is now unmasked. Its positive charge is added to the rest and the atomic number goes up by one. Thus, thorium-234 (atomic number 90) gives up a beta particle and becomes protactinium-234 (atomic number 91).

If a gamma ray is given off, that gamma ray has no charge and the equivalent of very little mass. That means that neither the mass number nor the atomic number of the nucleus is changed, although its energy content is altered.

Even more elaborate changes can be taken into account. In the long run, uranium-238, having gone through many changes, becomes lead-206. Those changes include the emission of 8 alpha particles and 6 beta particles. The 8 alpha particles involve a loss of 8 × 4, or 32 in mass number, while the 6 beta particles contribute nothing in this respect. And, indeed, the mass number of uranium-238 declines by 32 in reaching lead-206. On the other hand the 8 alpha particles involve a decrease in atomic number of 8 × 2, or 16, while the 6 beta particles involve an increase in atomic number of 6 × 1, or 6. The total change is a decrease of 16 - 6, or 10. And indeed, uranium (atomic number 92) changes to lead (atomic number 82).

80

It is useful to go into such detail concerning the proton-electron theory of nuclear structure and to describe how attractive it seemed. The theory appeared solid and unshakable and, indeed, physicists used it with considerable satisfaction for 15 years.

—And yet, as we shall see, it was wrong; and that should point a moral. Even the best seeming of theories may be wrong in some details and require an overhaul.

About HackerNoon Book Series: We bring you the most important technical, scientific, and insightful public domain books. This book is part of the public domain.

Isaac Asimov. 2015. Worlds Within Worlds: The Story of Nuclear Energy, Volume 2 (of 3). Urbana, Illinois: Project Gutenberg. Retrieved May 2022 from https://www.gutenberg.org/files/49820/49820-h/49820-h.htm#c19

This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org, located at https://www.gutenberg.org/policy/license.html.