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The expulsion of a j3-particle causes a shift of one place in the opposite direction. For example, by the loss of an a-particle from ionium of group IV., the resulting product, radium, belongs to group II., while the loss of another particle gives rise to the emanation which occupies the group 0, and so on. By this method the chemical properties of all the known radio-elements can be predicted from a knowledge of the radiations emitted from the products. This generalisation can be viewed from another im portant standpoint. From the work of Moseley, the properties of an element are defined by the atomic number which is be lieved to represent the resultant positive charge on the nucleus. The loss of an a-particle of mass 4, carrying two positive charges, lowers the atomic number by two units, while the emission of a 0-particle raises it by one unit. On looking through the table of the radio elements on preceding page it will be seen that many of them can be grouped under the same atomic number. These rep resent the radioactive isotopes of which some of the more im portant are given below, preceded by the atomic numbers :— 8i. Thallium (204), thorium C" (208), actinium C" (206).
82. Lead (207), uranium-lead (206), thorium-lead (208), radium D (210), thorium B (212), radium B (214), actinium B (210).
83. Bismuth (208), radium E (210), radium C (214), thorium C (212), actinium C (210).
84. Polonium (210), thorium A (216), radium A (218), actinium A (214) 86. Radon (222), thoron (220), actinon (218).
88. Radium (226), thorium X (224), mesothorium I (228), actinium X (222).
go. Thorium (232), radiothorium (228), ionium (23o), uranium Xi (234), uranium Y (23o), radioactinium (226).
will be seen that many of the radioactive elements are iso topic with known chemical elements. These radioactive isotopes differ not only in atomic weight but also in radioactive proper ties. The isotopes of lead are of special interest as they include the end-products of the uranium, thorium and actinium series— a question that will be discussed more fully later. It is of interest to note that polonium, actinium and protoactinium are new types of chemical elements which have no counterpart among the ordinary inactive elements.
In 190o the late Sir W. Crookes found that the a-ray activity of ordinary uranium could be removed by a single chemical operation and concentrated in an active residue. This is due to the separation of the product ura nium
of period 24 days, which emits 0-rays and 7-rays. A complete analysis of the transformations of uranium has been a matter of much difficulty. Boltwood showed that the a-ray activ ity of uranium was about twice as great as that of a corresponding a-ray product in the uranium-radium series, indicating that ura nium contained two successive a-ray products. This was con firmed by Geiger, who showed that the a-rays from uranium con sisted of two groups with ranges 2.5 and 2.9 cm. respectively. These two a-ray substances, called uranium I. and uranium II. are isotopic, atomic weights 238 and 234 respectively. The latter, whose period is estimated at about 2,000,000 years, exists in rela tively very small quantity compared with uranium I. Following the generalisation connecting the radiations and chemical proper ties of the series of radio-elements. Fajans predicted the presence of a new product with properties analogous to tantalum, and promptly succeeded in isolating it experimentally. The new prod
uct uranium X2, sometimes called brevium, has a period of 1.15 minutes and emits swift 0-rays. The series of changes is thus:— Antonoff discovered another j3-ray substance called uranium Y, separated with uranium
which has a period of 24.6 hours. This exists in too small quantity to be in the main line of succes sion, but is to be regarded as a branch product isotopic with ura nium
and is believed to be the first element of the subsidiary actinium series.
Rutherford and Geiger found the number of a-particles emitted per gram of uranium per second to be 2.37 X
From this the period of uranium is calculated to be about 6,000,000,000 years.
Radium is transformed di rectly into radon which in turn goes through a rapid series of transformations called radium A, B and C. The complete analysis of these changes has involved a large amount of work. Radon changes first into radium A, a substance of period 3 minutes emit ting only a-rays. Radium A changes into radium B, a product of period 26 minutes emitting a-rays of penetrating power small compared with those emitted from the next product radium C. The products included under the title radium C have proved of considerable importance, for they not only emit very penetrating a rays and fl-rays, but are the origin of the 7-rays arising from ra dium in equilibrium. We have already seen that radium C breaks up in a complex way. When a wire charged negatively has been ex posed for some time in the presence of radon, it becomes coated with an invisible film of radium A, B and C. After removal from radon for 20 minutes, radium A has practically disappeared and the a-rays arise entirely from radium C. Radium C has proved very valuable in radioactive measurements as providing an in tense source of homogeneous a-rays. Twenty-four hours after removal, the activity due to radium B and C has become exceed ingly small. There still remains, however, a very small residual activity, first noted by Mme. Curie. This residual activity meas ured by the a-rays rapidly increases with the time and reaches a maximum in about three years. The active deposit of slow change was examined in detail by Rutherford and by Meyer and Schweidler. It was shown to consist of three successive prod ucts called radium D, E and F. Radium D emits slow )5-rays and is half transformed in about 16 years. Radium D changes into E, a j3-ray product of period about 5 days, and E into F, an a-ray product of period 14o days. The product radium F is of special interest, for it is identical with polonium—the first active body separated by Mme. Curie. In a similar way it has been shown that radium D is the primary source of the activity observed in lead or "radiolead" separated by Hofmann. Radium D is a radioactive isotope of lead of atomic weight 210. It is always separated with the inactive lead present in a uranium mineral. It is interesting to note what valuable results have been obtained from an examination of the minute residual activity observed on bodies exposed in the presence of radon. On account of their special importance as sources of intense radiation a more detailed account will be given of radium and its origin, radon and polonium.