Thorium is peculiar, among the elements which have been considered, through its property of giving characteristic radiations, and disintegrating with formation of a whole family of new elements; or, as it is commonly expressed, through its radioactive properties.[474] The element has a half-life period of the order of 4 × 10¹⁰ years; in the course of decay, it gives rise to mesothorium 1, which is rayless, but decays to mesothorium 2, with its product radiothorium, both of which give powerful radiations. Mesothorium 1 of course occurs in all thorium-containing minerals, and may be separated from monazite by addition of a barium compound during the sulphuric acid decomposition; in consequence of the powerful radiating properties of its products, it is itself of considerable importance, and proposals for extracting it from monazite in the preparation of the thorium nitrate of commerce have been put forward (see [p. 276]).
[474] The nature of the present work allows only the briefest reference to be made to the exceedingly interesting phenomena which centre about this subject; for a more complete account, the student should consult Soddy, The Chemistry of the Radio-Elements, Part I, 1911.
Mesothorium appears to be chemically identical with radium; since monazite, like all other thorium-bearing minerals, contains uranium and radium, the latter element is separated with the mesothorium, and indeed, having a very much larger half-life period, constitutes by far the greater part of such ‘mesothorium’ preparations. On account of the great activity of the mesothorium products, the best preparations from monazite, though estimated to contain only 1 per cent. of mesothorium to 99 per cent. radium, are said to be four times as active as pure radium compounds. The chemical identity of the two products seems to preclude any possibility of determining the physical properties and constants of mesothorium.
The element radiothorium, which was discovered by Hahn in 1905, in the mineral thorianite, is chemically identical with the parent element thorium, but can be separated by means of the intermediate element, mesothorium 1. The latter is readily separated by the sulphate precipitation, and the radiothorium to which it gives rise may be separated by precipitation with ammonia. Thorium is also chemically identical with ionium, the parent of radium, and the thorium nitrate of commerce therefore contains important quantities of ionium—important that is, in view of the high radiating power of the latter element. The study of these relationships constitutes one of the most important and interesting fields in the province of radioactivity.
The Metal.—Elementary thorium has not yet been obtained in the pure state, owing to the ease with which it forms compounds and alloys with all the common elements, and to its great affinity for oxygen; the high melting-point also increases the difficulty of obtaining the pure metal. Berzelius attempted to reduce the alkali double fluorides and double chlorides with sodium or potassium; Nilson carried out the same reaction in a closed iron cylinder, but his product still contained 20 per cent. of thoria. Reduction of the oxide with magnesium is never complete, and the carbon method gives only a mixture of carbide and metal. Electrolytic methods give no better results, since the metal liberated at the cathode always encloses oxide and other impurities. Moissan and Hönigschmid in 1906, by heating the carefully purified anhydrous chloride with sodium in a sealed glass tube from which air and moisture had been removed, claim to have obtained a product containing only 3 per cent. of the oxide. The element has recently been prepared in leaf form by forcing the amorphous product into the bore of a copper tube, hammering into sheets, and removing the copper by dilute nitric acid.[475]
[475] v. Bolton, Zeitsch. Elektrochem. 1908, 14, 768.
The amorphous impure metal is a dark grey powder, of specific gravity 11·3; the hammered and strongly heated leaf has the density 12·16. It burns readily in air with great brilliance, and when finely powdered ignites if crushed or rubbed. When heated in the electric furnace, it melts, according to von Bolton,[476] at about 1450°; von Wartenburg[477] found the melting-point to be about 1700°; the fused beads resemble platinum in physical properties. It is somewhat resistant to acids, dissolving easily only in aqua regia, and more slowly in fuming hydrochloric acid. It combines directly when heated in sulphur or halogens, and in nitrogen and hydrogen.
[476] v. Bolton, Zeitsch. Elektrochem. 1908, 14, 768.
[477] Ibid. 1909, 15, 866.
The hydride, ThH₄, is best obtained by heating the metal in hydrogen, an energetic reaction taking place at a red heat. Winkler observed that a mixture of the dioxide with magnesium absorbs hydrogen readily when heated. The hydride is a stable greyish-black powder, not attacked by water, but dissolving readily in hydrochloric acid, with evolution of hydrogen. The nitride, Th₃N₄, is prepared by heating the metal in the gas, or the carbide in a stream of ammonia. It is a brown powder, decomposed by water with evolution of ammonia and formation of the dioxide. The azide has been used for purposes of detection and estimation, since in boiling solution it is hydrolysed with separation of the hydroxide; zirconium and ceric salts also show this reaction, but the rare earth salts give no precipitate.