Uranium is a hard, dense, malleable, ductile, silver-white, radioactive metal of the actinide series in Group 3 of the periodic table. Uranium has three distinct forms (see allotropy); the orthorhombic crystalline structure occurs at room temperature. It is a highly reactive metal and reacts with almost all the nonmetallic elements and their compounds, especially at elevated temperatures. It dissolves readily in nitric and hydrochloric acids but resists attack by alkalies. It forms solid solutions and intermetallic compounds with many of the metals. Metallic uranium tarnishes in air and when finely divided ignites spontaneously.
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