of the same nuclear charge but of' different atomic masses. As we have seen, the nuclear charge controls the ordinary physical and chemical properties of the atom, and the mass which resides almost entirely in the nucleus has only a second-order effect. On the other hand, the property of radioactivity depends on the structure and stability of the nucleus, which may be very dif- ferent for atoms of the same resultant nuclear charge.
In the article on RADIOACTIVITY attention is drawn to the remarkably simple relation which exists between the chemical properties and radiations of the series of radioactive elements. With the aid of this relation we can at once write down the ordinal numbers and masses of the long series of elements which arise from the transformation of uranium, thorium and actinium, and can follow the origin of the numerous isotopes which arise. One of the most striking results of this generalization was the predic- tion that the end product of the uranium and thorium series should be an element of the same ordinal number as lead but of atomic masses 206 and 208 respectively, instead of the mass 207 found for ordinary lead. This result has been directly confirmed by atomic weight determinations of uranium-lead and thorium- lead, and was the first definite proof of the existence of isotopes of a non-radioactive element.
It seemed probable that in a similar way many of the ordinary elements might consist of a mixture of isotopes, i.e. elements with the same nuclear charge but different atomic masses. This has been confirmed in a number of cases chiefly by the work of Aston. The masses of the positively charged atoms present in the electric discharge in a vacuum tube are examined by bending the rays in a combined magnetic and electric field. In this way it was found that neon consisted of two isotopes of masses 20 and 22 and chlorine of isotopes of masses 35 and 37. The relative pro- portions of the two isotopes in chlorine was in good accord with that to be expected from the ordinary atomic weight of the mixture of isotopes, viz. 35.45.
This new method of analysis had, up to 1921, been employed only for a small number of the elements, but had yielded re- sults of great interest. Some of the elements, like carbon, nitro- gen and oxygen, give no isotopes, and are thus to be regarded as " pure " elements where the atoms have all the same mass and nuclear charge. Others, like chlorine, argon, krypton, and mer- cury, are composed of a mixture of two or more isotopes. In cases like krypton and mercury as many as six well-defined isotopes have been detected. As far as observation has gone the masses of all the isotopes are expressed by a whole number in terms of O= 16 with an accuracy of about i in 1,000. For example, the isotopes of neon are 20.00 and 22.00. This important con- clusion, which has been verified in a number of cases, affords a strong indication that the masses of the parts composing the nucleus have a mass either of one or a multiple of one, and are not direct multiples of the mass of the hydrogen atom which is 1.008 where O = 16. The reason of this will be discussed later.
While the ordinary physical and chemical properties of iso- topes are closely similar, it is to be expected that they should differ in all qualities which involve directly the mass of the atom, e.g. the coefficients of diffusion and specific heats. In a similar way second-order effect is to be expected in the rate of vibration of the external electrons, i.e. in the light spectrum of the element, and a small effect has been observed in several cases. The most obvious method of partial separation of isotopes is by the process of diffusion or evaporation. In this way a partial separation into light and heavy fractions has been shown in the case of neon, mercury, and chlorine. No evidence of the separation of isotopes in nature has been so far observed except in the case of uranium- lead and thorium-lead already referred to. It will be of great interest to test, for example, whether chlorine obtained from widely different sources shows any difference in the relative proportions of its component isotopes.
Distribution of Electrons. We have seen that the atom is to be regarded as an electrical structure in which a positively charged nucleus is surrounded by a number of electrons. The magnitude of the nuclear charge and the number of the external electrons are known for each of the elements. In considering the distribution
of the external electrons round the nucleus, we are at the outset faced by the great difficulty that no possible arrangement can be permanently stable on the basis of the classical dynamics. For example, an electron in motion round the nucleus must on the classical theory radiate energy and fall into the nucleus. To overcome this fundamental difficulty Bohr has introduced a conception based on the quantum theory, in which radiation only occurs in definite quanta. In this way it is possible to postulate the position of the electrons in the simpler atoms and to calculate their frequency of vibration. The theory of Bohr developed by Sommerfeld and others has achieved remarkable success in explaining many of the details of the spectra of hydro- gen and helium both in electric and magnetic fields. Owing, how- ever, to the great complexity of the possible modes of motion when three or more electrons are present, it is difficult to calcu- late the distribution of the electrons and their modes of vibration in the case of the more complex atoms.
A number of suggestions have been made as to the grouping of the electrons in the atom, notably by Kossel, Lewis, Langmuir and Sir J. J. Thomson, which have had a certain measure of success in offering an explanation of the periodic variation in the properties of the elements with atomic number and the methods of combination to form molecules. These theories, however, are for the most part descriptive and not quantitative in charac- ter. The whole problem of the distribution and motion of the electrons in a complex atom is a very difficult one. While def- inite progress had been made by 1921, much still remained to be done before we could hope to define with any certainty the position, motion and modes of vibration of the electrons for even the lighter and less complex elements.
Structure of the Nucleus. While it is difficult to estimate the dimensions of atomic nuclei, the general evidence indicates that the nucleus of a heavy atom like uranium, if assumed spherical, has a radius of less than io u cm. or less than i/iooo of the radius of the external atom. No doubt the dimensions of a nucleus depend on its complexity and are much smaller for the lighter atoms. From experiments on the passage of a particles through hydrogen, it has been calculated that the dimensions of the helium nucleus of mass 4 is of the order io 12 cm.
The most direct evidence on the constitution of the nucleus is derived from the study of the radioactive transformations. The disintegration of an atom is accompanied either by the expulsion of an a particle, i.e. in helium nucleus, or the release of a swift electron from the nucleus. This shows that the nucleus of the radioactive atoms contains both positively charged masses and negative electrons, and that the nuclear charge represents the resultant charge. It is natural to conclude that the helium nu- cleus of mass 4 is one of the secondary units which make up the structure of a complex nucleus. This is supported by the ob- servation that the atomic mass of many atoms is expressed by 4 where n is a whole number. It is clear, however, from the work of Aston on isotopes that, in addition to the helium nucleus, an element of mass i or integral multiple of i enters into the structure of all nuclei. This fundamental unit of structure has been named " proton," and its atomic mass is i or very nearly i in terms of O= 16. On this view the nuclei of all elements are made up of positively charged protons and electrons. The mass of the atom measures the number of protons in the nucleus. This is in a sense a return to the famous hypothesis of Prout in which all the atoms are supposed to be built up of hydrogen as the fundamental unit.
It seems clear that if a proton could be removed from an atomic nucleus it would prove to be the hydrogen nucleus carry- ing a unit positive charge. In fact, Rutherford and Chadwick have shown that the hydrogen nucleus can be liberated from certain atoms like nitrogen and aluminium by bombardment with swift a particles. It remains, however, to explain why the proton in a nucleus has a different mass from the free hydrogen nucleus. The latter has a mass 1.008 in terms of O= 16 while the proton in the nucleus has a mass unity, or nearly unity.
While the negative unit of electricity exists in the form of the electron of very small mass, no evidence has been obtained that
