II.

Properties

Uranium melts at about 1135°C (about 2075°F), boils at about 4131°C (about 7468°F), and has a specific gravity of 19.05 at 25°C (77°F); the atomic weight of the element is 238.03. Uranium has three crystalline forms, of which the one that forms at about 770°C (about 1418°F) is malleable and ductile. Uranium is soluble in hydrochloric and nitric acids, and it is insoluble in alkalies. Uranium displaces hydrogen from mineral acids and from the salt solutions of such metals as mercury, silver, copper, tin, platinum, and gold. When finally divided, it burns readily in air at 150° to 175°C (302° to 347°F). At 1000°C (1832°F), uranium combines with nitrogen to form a yellow nitride.

Uranium has oxidation states of three, four, five, and six. The hexapositive compounds include uranyl trioxide, UO₃, and uranyl chloride, UO₂Cl₂. Uranium tetrachloride, UCl₄, and uranium dioxide, UO₂, are examples of the tetrapositive, or uranous, compounds. Uranous compounds are usually unstable; they revert to the hexapositive form when excessively exposed to air. Uranyl salts, such as uranyl chloride, may decompose in the presence of strong light and organic matter.