Sequence of Changes of Uranium (U) and Thorium (Th) into various Isotopes of Lead (Pb.)

[Face page 134.

CHEMICAL CHARACTER

THE CHEMICAL CHARACTER OF THE RADIO

ELEMENTS.

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The simple connection between the sequence of radioactive changes and the chemical character of the products has effected an enormous simplification, not only in the theory, but also in the practice of radio-chemistry. The series extends over twelve places, two, namely those in the families of the halogens and the alkali metals, being entirely skipped. In the ten occupied places are forty-three distinct types of matter, but only ten chemical elements. Seven of these ten, thallium, lead, bismuth, emanation, radium, thorium, and uranium, can now in every respect be considered, both chemically and spectroscopically, thoroughly well known. These seven places accommodate all but nine of the known radio-elements, and these nine, the isotopes of polonium, actinium, and ekatantalum respectively, are the only members the chemistry and physics of which cannot be referred to well-known elements obtainable in sufficient quantity for ordinary chemical and spectroscopic examination.

Of these three, polonium, although the element of which at present the chemistry is best known, is likely to remain the most difficult to bring into line with the others, for, although a vast amount of exact information has been obtained as to its reactions, it would seem to remain hopeless ever to obtain it in anything but infinitesimal amount owing to its relatively very short period.

The chemistry of actinium has been enormously simplified by the discovery that mesothorium-2 is isotopic with it, for the latter may be used as an indicator to show in what way the actinium distributes itself after any chemical treatment. Owing

to its relatively small quantity as a branch product and to the fact that, itself, it gives no rays, the characteristic radioactivity of its products only making their appearance slowly after it has been separated, actinium has always been a difficult element to extract from the mineral and very easy to lose in chemical operations. There is now, however, another reason which will assist in the study of this element.

THE ORIGIN OF ACTINIUM. EKATANTALUM.

The generalisation has now led to the elucidation of its origin and the discovery of its direct parent. From its constant association with uranium minerals, and the relative activity therein of its products in comparison with the activity of those of radium, it was considered to be a branch product of the uranium series, only 8 per cent. of the atoms of uranium disintegrating passing through the actinium series and 92 per cent. through the radium series. Its definite location in the periodic table, by virtue of its isotopism with mesothorium-2, made it clear that its parent must either be in the radium or the ekatantalum place, the former if it is produced in a B-ray change and the latter if it is produced in an a-ray change.

The ekatantalum place was vacant when the generalisation was first made, but it was necessary to suppose that uranium-X, like mesothorium, comprised two successive products, uranium-X1 and uranium-X, both giving B-rays, and the latter occupying the vacant place in question. This prediction was confirmed within a few weeks of its being made by the discovery by Fajans and Göhring of uranium-X2, or brevium, a new member responsible for the more penetrating B-radiation given by uranium-X,

ORIGIN OF ACTINIUM

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The

and having a period of only 1.65 minutes. possibility that actinium was produced in a ß-ray change from an isotope of radium was experimentally disproved, and there remained only the second alternative, which was rendered the more probable by the existence of a member, uranium-Y, discovered by Antonoff, isotopic with uranium-X1, and simultaneously produced with it from uranium in relative quantity such as is to be expected, if it were the first member of the actinium series. Uranium-Y, like uranium-X1, gives soft B-rays, and hence its unknown product must be the isotope of uranium-X2, and might also well prove to be the unknown direct parent of actinium in an ɑ-ray change of long period.

During the year the missing element has been found in two independent investigations (Soddy and Cranston, Proc. Roy. Soc., 1918, [A], 94, 384; O. Hahn and L. Meitner, Physikal. Zeitsch., 1918, 19, 208). The problem as it presented itself to us was so to treat a uranium mineral as to separate an element, if present, which possessed the chemical character of the known but hopelessly short-lived uranium-X2, using the latter as an indicator in trying possible methods beforehand. The method adopted, distillation at an incipient red heat in a current of carbon tetrachloride vapour and air, was found to be very effective in volatilising uranium-X2 from uranium-X1, and when applied to pitchblende it was found to give a product in which none of the known pre-emanation members of the disintegration series were present. Thus was obtained a preparation from which actinium was at first absent, but which, with the lapse of time, continuously generated actinium, as characterised beyond the possibility of doubt by means of its active deposit.

It should be mentioned that the exact point at