The Discovery of Subatomic Particles by Steven Weinberg

Author:Steven Weinberg [Weinberg, Steven]
Language: eng
Format: epub
ISBN: 9780716714880
Google: F1TCQgAACAAJ
Published: 2022-08-08T01:43:29+00:00

4 The Nuclens

1 0 1

of bison peacefully graze. I t is sometimes asked why physicists today need hundreds of millions of dollars for giant accelerators when Rutherford accomplished so much on a table top. I think the answer is that those discoveries on the fundamental nature of matter that can be made with string and sealing wax have mostly been made, in large part by Rutherford.

So far, I have emphasized the problem of the distribution of electric charge in the atom, but the work of the Rutherford group at Manchester settled another question that had been raised by Thomson’s discovery of the electron: What is the distribution of mass in the atom? As we saw in Chapter 3, the work of John Dalton and other chemists in the early nineteenth century had determined the relative masses of the atoms of different elements, revealing that the carbon atom weighs 1 2 times as much as the hydrogen atom, the oxygen atom weighs 1 6 times as much as hydrogen, and so on. Also, the work on electrolysis by Faraday and others had shown that the electrically charged atoms (ions) that carry electric currents in solutions of acids or salts have a mass/charge ratio of about 1 0-8 kilograms/coulomb for hydrogen, and proportionally more for the ions of heavier atoms. After the discovery of the electron, it became fairly clear that these ions are nothing but atoms that have gained one or more electrons (for negatively charged ions) or lost one or more electrons (for positively charged ions). On this basis, the electric charge of the hydrogen ion should be just equal in magnitude to the charge of the electron. Thus, since the mass/charge ratio of the electron is 1/2,000 that of the hydrogen ions, and the charges are equal, the mass of the hydrogen ion (and atom) should be some 2,000 times larger than the mass of the electron. Does this mean that atoms consist of thousands of electrons? Or is most of the mass of the atom somewhere else, perhaps associated with the positive electric charges?

As we will see later, the 1 909- 1 1 Manchester experiments showed not only that the positive electric charge of the atom is concentrated in a tiny nucleus, but also that this nucleus contains almost all the mass of the atom. What then does the nucleus consist of? Dalton found that atoms have masses generally close to multiples of the mass of the hydrogen atom, so one might think that the nuclei of atoms consist of heavy, positively charged particles that can be identified with the nucleus of hydrogen, particles that Rutherford in 1 920 named protons. However, Rutherford’s own results showed that this would not work; for instance, the mass of the helium nucleus is four times that of hydrogen. but Rutherford found that its electric charge was only twice as great. As we shall see, Moseley measured other nuclear charges in 1 9 1 3 and found the same pattern -

for instance. calcium has an atomic weight 40 times that of hydrogen, but a nuclear charge only 20 times as great.

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