Characteristics: Amorphous red powder, becoming black on standing and crystalline on heating; vitreous and colloidal forms may be prepared. Amorphous form softens at 40 °C and melts at 217 °C. It rarely occurs in its elemental state in nature, or as pure ore compounds.
Crystalline selenium is a p-type semiconductor and electrically, it acts as a rectifier, and has marked photoconductive and photovoltaic action (converts radiant to electrical energy). The electrical conductivity increases with increasing light irradiation.
Soluble in concentrated nitric acid and in (liquid form) common alkalies. Forms binary alloys with silver, copper, zinc, lead, etc. Low toxicity. A necessary nutritional factor for animals.
Hazards: Selenium salts are toxic in large amounts, but trace amounts are necessary for cellular function.