Atomic number: 34

Symbol: Se

Category: metalloid sometimes non-metal

Density at 25°C: solid:  from 4.39 to 4.81 g·cm−3

Trace amounts of selenium is needed for most life. It acts mainly as part of anti oxidising enzymes. It is also used in glass making and pigments.

Selenium black form. image: adapted from wikipedia

Use, natural applications or common combinations: 

  • Enzymes
  • Solar panels
  • The substance loosely called selenium sulfide (approximate formula SeS2) is the active ingredient in some anti-dandruff shampoos. The selenium compound kills the scalp fungus Malassezia, which causes shedding of dry skin fragments. This was mentioned in a comical way in the movie Evolution(imdb link).
  • Used in brasses with bismuth and selenium instead of toxic lead.

Basic characteristics:

  • Standard atomic weight: 78.96(3)
  • Melting point: 221 °C
  • Boiling point: 685 °C
  • Radioactivity: almost all stable
    • 79Se trace amounts with half-life of 3.27×105 y