Although it is a halogen, iodine has a metallic look and sometimes is seen as a metalloid. It is the element with the highest number that is essential to multicellular organisms, humans process it in their thyroid gland. Iodine is less reactive than the other halogens. It is quite volatile and easily evaporates to purple I2 gas. Several highly radioactive isotopes of iodine, which have half-lifes of some hours and days, occur as fission products from uranium and plutonium. Due to their accumulation in the thyroid gland, these are amongst the biggest threats after nuclear accidents.
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