Atomic number: 53

Symbol: I

Category: halogen

Density at 25°C: solid:  4.933 g·cm−3

Although iodine is more common in the oceans, on the land it is sometimes quite rare on the crust of the Earth. Because iodine is critical to many land animals including humans, deficiencies can sometimes occur that lead to thyroid problems.

Iodine crystals. image: adapted from wikipedia

Use, natural applications or common combinations: 

  • essential in thyroid function
  • antiseptics
  • radioactive isotopes for medical imaging.

Basic characteristics:

  • Standard atomic weight: 126.904 47(3)
  • Melting point:  113.7 °C but appears to sublimes easily to a purple gas.
  • Boiling point: 184.3 °C
  • Radioactivity: most iodine in nature is stable 127I  , but there are a lot of radioactive isotopes trace amounts usually from fission sources, or synthetically made.