The atomic mass of radium is about 226. When an atom of it explodes, an alpha particle with a mass of 4 is shot out. The atom of the element called radium emanation, which results from this explosion, should then have an atomic mass of about 222. Thus, whenever an atom in disintegrating loses an alpha particle, its atomic mass decreases by 4. If it loses a beta particle, there should be no appreciable change, for the mass of a beta particle is too small to produce any measurable effect. On the whole, experimental evidence supports these explanations.

In the transition from uranium to lead there are eight transformations in which an alpha particle is shot out; hence there should be a total loss in atomic mass of 8 × 4 = 32. The accepted value of the atomic mass of uranium is 238. Subtracting 32 from this we get 206, the atomic mass of lead which has descended from uranium. The atomic mass of ordinary lead is 207.2. It is now known that there is more than one kind of lead, and that ordinary lead is a mixture. (The different kinds of lead have identical chemical properties, but different atomic masses.) Lead found in radioactive ores has an atomic mass of 206.3. This agrees well with the theoretical value, 206, given above.

544. Source of the gamma radiation. When a stream of cathode rays strikes any substance, X-rays are emitted by that substance. In a similar manner, when a stream of beta rays (electrons of higher speeds than those given off from a cathode) strikes any substance, it causes the substance to emit a gamma radiation, which is a very penetrating form of X-rays. Since the beta radiations from a radioactive substance originate not only at the surface but all through the interior of the mass, a large percentage of the beta particles ejected from the atoms strike adjacent atoms of the radioactive substance, producing oscillations in these atoms which cause them to emit the wave-motion called gamma rays. High-frequency waves should also be caused by the emission of a beta particle from the nucleus of an atom; for undoubtedly violent oscillations within the atom must be produced. Hence we should expect gamma radiation in all those transformations where beta rays are emitted. This is found to be the case.

545. Structure of an atom. It has been stated several times in this book that an atom is believed to consist of a positively charged nucleus with negatively charged electrons clustered around it. This conclusion has been reached largely through a study of the radioactive properties of substances and their radiations.*

The fact that the alpha and beta particles are shot out with so much energy indicates that they originate in the nucleus. Since the alpha particles from all radioactive substances have the same mass (that of the helium atom), and the beta particles are always electrons, it follows that the central part of the atom is probably made up of an assemblage which contains positively charged alpha particles and negatively charged electrons.† But there must be an excess of positive charge in the nucleus. Surrounding the nucleus are electrons. The number of these is determined by the

* Confirmation is found in the study of the spectra of light and X-rays.

† To account for the fact that the masses of the different kinds of atoms are not always an integral multiple of the mass of the alpha particle, it is usually assumed that the nuclei of some elements contain also hydrogen nuclei. There is some evidence in favor of this assumption. Rutherford recently has been apparently able, by using swift alpha particles, to knock hydrogen nuclei out of the nuclei of a number of different kinds of atoms. All these atoms have a mass which is not an integral multiple of the mass of an alpha particle.

magnitude of the excess of positive charge on the nucleus. In an uncharged atom there must be just enough electrons to neutralize the positive charge on the nucleus; for example, if the excess positive charge is 20 natural units, then there must be normally 20 electrons, since each carries 1 natural unit of negative electricity. It has thus been possible to estimate the number of the surrounding electrons. It is generally believed that the hydrogen atom has only 1, and that the number for other elements is about half their atomic mass and equal to the "atomic number," which is explained in the next section.

Certain experiments indicate that the diameter of an atom is about 10-8 centimeters and that the diameter of an electron is about 10-13 centimeters. The diameter of the nucleus of a gold atom, which is relatively massive, is not over 10-12 centimeters. The diameter of the nucleus of a hydrogen atom is estimated to be about 10-16 centimeters.

546. Atomic numbers. As is well known, there is a certain progressive change in the properties of chemical elements. In texts on chemistry one will usually find a table with the elements arranged in accordance with the "periodic law." If these elements are numbered consecutively hydrogen 1, helium 2, lithium 3, etc., numbers will be obtained which are called the atomic питbers. In most cases these numbers follow the order of the atomic masses.

The character of X-ray radiation given off from any element struck by a stream of high-speed electrons has been found to depend on the atomic number of that element; in fact, a study of the characteristics of the X-rays emitted by different elements gives the most accurate method of determining the atomic number of many elements.

According to current theories of the structure of the atom, the atomic number of any element is always equal to the number of natural units of the excess of positive electricity on the nucleus of the atom of that element. This is also equal to the number of electrons surrounding the nucleus when the atom is in electrical equilibrium, because in that case there must be a sufficient number of electrons to neutralize the positive charge on the nucleus.

547. Isotopes. The chemical properties of an element depend on the atomic number of that element, and this apparently depends on the electrical charge on the nucleus. Since the charges carried off by the radiation in the disintegration of radioactive substances are known, it is possible to compute the changes in the charge on the nucleus. These changes are traced out in the following table for a portion of the uranium-radium series.

The assumption is made, in compiling this table, that the charge on the nucleus of radium A is equal to the atomic number 84. With this assumption one can, by the use of the table, readily arrive at the other nuclear charges, since the charge carried away from an atom by the radiation is known. Two things become evident from the table:

1. There is a confirmation of the theory that the atomic number is equal to the nuclear charge, for the computed nuclear charges in this table agree with the values of the atomic numbers which are known.

2. Certain of these elements have the same nuclear charge and the same atomic numbers. Radium B, radium D, and lead have an atomic number 82. They are found to have identical chemical properties. Radium A, C1, and F have an atomic number 84. They too have the same chemical properties. Elements having the same chemical properties are called isotopes.

It has been recently found by Aston that there are a large number of isotopes. Thus chlorine is really a mixture of two gases: one having an atomic mass of 35.0, the other having an atomic mass of 37.0. These two gases have identical chemical properties. It is now believed that many of the so-called elements are really mixtures of isotopes.

548. Other radioactive series. The uranium-radium-lead series is not the only known series of elements that are continuously disintegrating. Actinium, which is probably another descendant of uranium, gives rise to a number of other elements. Thorium and its descendants apparently form a series independent of that of uranium. They have about the same range in atomic masses and atomic numbers as those of the uranium-radium series, and have as the end product lead with an atomic mass of 208, -an isotope of the lead which is the end product of the uranium-radium series. Both the actinium and the thorium series give out the same kind of radiations as those of the uranium-radium series.

CHAPTER XXXIX

HIGH-FREQUENCY OSCILLATIONS AND ELECTRICAL

WAVES

A mechanical oscillating system, 549. An electrical oscillating system, 550. A method of producing a series of damped oscillations, 551. Electrical resonance, 552. Undamped high-frequency oscillations, 553. Electromagnetic waves along wires, 554. Electromagnetic waves in space, 555. The detection of electromagnetic waves, 556.

549. A mechanical oscillating system. Let us first consider the case of something which we know will oscillate when disturbed from its position of rest. As a simple case take a mass suspended by a spiral spring as shown in Fig. 340. If the mass is raised or lowered and then released, it will move up and down with an oscillating motion. A little thought will show us that there are three necessary conditions which must be fulfilled in order that the mass may oscillate.

1. There must be a return force; that is, one which tends to restore the mass to its original position. In this case the force is furnished by the spring and by gravity.

2. The moving parts must have inertia; for otherwise the mass would not move past its position of equilibrium but, when released, would move immediately to this position and stop.

FIG. 340

3. The friction must not be too large. If the mass were hung in a viscous fluid, it might not oscillate, but might slowly return to its position of equilibrium.

These three conditions hold not only for the foregoing case but for all types of oscillation. If, for any kind of contrivance, these three conditions are met, then we may be certain that there will be oscillations whenever the equilibrium is disturbed.