Uranium Compounds

Uranium is the terminal member of the chromium group of elements, and in its chemical behaviour it shows considerable resemblance to the other members of the group. Like them Uranium in Uranium Compounds valency varying from 2 to 6, but it is most stable in the hexavalent condition. The trioxide, UO₃, like the trioxides of molybdenum and tungsten, is amphoteric, but the acidic qualities are less evident. It forms on the one hand a hexafluoride, UF₆, and a large group of uranyl salts of the type UO₂X₂, where X is a monovalent electronegative element or radical; and on the other, stable uranates corresponding to acids of composition H₂UO₄ and H₂U₂O₇. It does not, however, tend to the formation of complex heteropolyacids in any marked degree. The uranyl salts, which contain the uranyl group UO₂ as a divalent metallic radical, may be regarded as stable intermediate products of the hydrolytic decomposition of the normal salts of hexavalent uranium. They are well-defined crystalline salts, generally yellow, although the presence of certain organic groups in the molecule may displace the colour strongly towards the red; both in the crystalline form and in solution they exhibit a greenish fluorescence. They are somewhat analogous to basic salts, but are stable in solution and little hydrolysed. They enter into combination with other salts, especially those of organic acids, and form numerous stable complexes of well-defined crystalline form. Of the normal uranic salts only the hexafluoride is known.

The uranous compounds correspond to the basic oxide, UO₂, and are usually green or blue in colour. They may be prepared by reduction of uranyl salts in solution under the influence of light (see below), but they are the more unstable and the solutions are readily oxidised back by air to uranyl compounds, especially in the presence of platinum black or of salts of iron or copper. In the uranous compounds uranium shows considerable chemical similarity to thorium, the terminal member of Group IV.

The intermediate oxide, U₃O₈, which appears to contain both tetra- and hexa-valent uranium, does not yield a corresponding series of salts. A large number of violet precipitates have, however, been obtained from solutions containing uranous and uranic salts present together, all of which, by loss of their acid radical, tend to become transformed into the green hydrated oxide, U₃O₈.2H₂O.

In addition to the derivatives of the oxides, UO₂ and UO₃, uranium forms a few compounds in which it appears to exhibit valencies equal to 5, 3, and possibly 2. It yields, for example, a comparatively stable pentachloride, UCl₅, and a much less stable trichloride, UCl₃. It produces a variety of sulphides: US, U₂S₃, US₂, UO₂S, but it does not show the tendency found in molybdenum and tungsten to form soluble sulpho-salts.

It forms a peracid, UO₄.2H₂O, which shows considerable activity towards metallic peroxides, with the production of a well-defined series of peruranates.