Iodine is a chemical element with the symbol I and atomic number 53. It was discovered in 1811 by Frenchman Bernard Courtis in one of those luck accidents when he added too much sulfuric acid to ashes of seaweed that he was processing. The purple vapor that was given off crystallized out on nearby surfaces and was eventually given the name “iodine” derived from the Greek word for purple.
Iodine is a non-metallic element of the halogen family in the periodic table which also includes fluorine, chlorine, bromine and astatine. It is a bluish black solid under normal conditions but it does sublimate into purple vapor. It melts at 113.7 °C but also gives off the purple gas as it melts. It dissolves easily in most organic solvents but does not dissolve very well in water unless it is in the presence of potassium. Iodine is a relatively rare element which is found in greater concentrations in free ions of iodine salts in ocean water than in rocks. It is rare in soils and is leached out by rainwater and carried to the ocean.
Iodine is the heaviest of the essential mineral elements and was probably incorporated into biological functioning because of its presence in the oceans where life originated. Iodine assists in the synthesis of thyroid hormones and it is absorbed by the thyroid when it enters the body. Because of its rarity in soil, especially away from the ocean, about two billion people on earth have iodine deficiency which can lead to intellectual disabilities.
Iodine has 37 isotopes from atomic weight of I-108 to I-144. Only one of the isotopes, I-127 is stable and the rest are radioactive. The half-lives vary from I-130m2 with a half-life of 133 billionths of a second to I-129 with a half-life of 15.7 million years. All the rest of the radioisotopes of iodine have half-lives of less than 60 days. All most all of the Iodine on earth occurs is in the form of the stable isotope. The tiny amount of I-129 on earth results mostly from nuclear explosions and nuclear accidents. Other isotopes rapidly vanish after their creation.
I-135 is produced in nuclear reactors and decays to xenon-135 which “poisons” nuclear fuel by reducing the neutron flux and slowing fission reactions. Large amounts of Xe-135 can temporarily interfere with restarting a shut-down reactor.
I-131 is one of the primary by-products of nuclear fission and is produced in nuclear reactors. It emits highly energetic beta particles and is the most carcinogenic of all the isotopes of iodine. When it escapes into the natural environment by nuclear explosions or severe nuclear accidents, it poses a very serious but short term health danger. Nonradioactive iodine in the form of potassium iodine tablets is given to people who may be exposed to I-131 because it will saturate the thyroid gland and prevent the I-131 from being absorbed.
Four of the isotopes of iodine are produced commercially for medical use. The beta radiation emitted by I-131 is used to kill thyroid tissue to prevent it from becoming cancerous. Isotopes I-123 and I-125 emit gamma rays which are useful as tracers which help image the structure and functioning of the thyroid gland. I-125 can also be implanted in a small capsule to irradiate cancerous tissue in the lungs. I-124 emits protons which is useful for positron emission tomography (PET) for direct imaging of the thyroid gland or tracer imaging.
Iodine crystals:
