Selenium

Selenium (atomic no. 34) is a member of Group VI, also known as the S family, in the periodic table. It has specific gravities of 4.79 g/cm3 for the metallic (gray) form, or 4.28 g/cm3 for the vitreous (black) form. It has an atomic weight of 78.96, melting point of (gray form) 217°C, and boiling point of (gray form) 684.9 ± 1°C. Because of its chemical similarity to S, it resembles S both in its forms and compounds and accounts for their many interrelations in biology. It has six stable isotopes in nature with the following percentage isotopic composition: 74Se, 0.87; 76Se, 9.02; 77Se, 7.58; 78Se, 23.52; 80Se, 49.82; and 82Se, 9.19. The most important oxidation states of Se are — II, II, IV, and VI. Selenium can be easily oxidized from Se(0), elemental Se, to Se(IV), (SeO₃2−), and to Se(VI), (SeO₄2−). Selenites (SeO₃2−) are stable in alkaline to mildly acidic conditions and can be found in nature. Some of the known commercial Se compounds are H₂Se, metallic selenides Se(— II), SeO₂, H₂SeO₃, SeF₄, Se₂C1₂, and H₂SeO₄ (selenic acid). Selenium also forms a large number of organic compounds that are analogous to those of S.