What do Fukushima, mudpuppies and the colour purple have in common?

This week's element, iodine, is probably much more familiar to you than some of the previous ones we've talked about recently. Iodine is designated by the symbol, I, and the atomic number, 53, and its name comes from the Greek word for violet or purple, which is the colour of elemental iodine vapor.

Purple? you ask.

Yes, purple. This character alone makes iodine fun since not only is it a departure from the usual variations on the grey colour theme for most elements, but it is a radical departure into purple. In addition, crystals of elemental iodine are quite attractive, being black and shiny.

Iodine is used industrially as a catalyst to speed up other chemical reactions, particularly the production of commercial quantities of acetic acid (vinegar), as well as having a variety of applications in the pharmaceutical industry. But iodine's primary importance is biological: it is an essential element for animals. The thyroid gland synthesizes several hormones, triiodiothyronine (T3) and thyroxine (T4), which contain the element. For this reason, and also because iodine is fairly rare, the thyroid gland concentrates iodine. Normally, this is a good thing, but this can pose a problem when the body is flooded with radioactive isotopes of iodine. Accumulations of iodine radioisotopes, particularly iodine-131, which has a half-life of 8 days, are carcinogenic to the thyroid gland. This is the reason for providing the public with potassium iodide tablets after a nuclear disaster such as the Fukushima accident, because providing the body with a large influx of the nonradioactive I-127 isotope saturates the thyroid gland so it cannot absorb radioisotopes of iodine.

Iodine is present as water-soluble ions in seawater, sea salt, and in a number of marine plants, particularly kelp. But far from the coasts, in inland and mountainous regions of the world, iodine deficiency is a preventable public-health tragedy. Since the thyroid hormones, T3 and T4, regulate the genes that orchestrate growth, development, and metabolism in animals, iodine deficiencies have serious consequences: it is the leading preventable cause of mental retardation, goiter (known as "Derbyshire Neck" in parts of the UK) and it contributes to development of a number of cancers. These public health issues can be easily remedied by adding iodine to salt, flour, water and milk in areas of deficiency. Further, the cost of adding iodine to foods is negligible, but the social aftermath of not doing so are enormous, leading to billions of dollars in health care expenditures every year. Surprisingly, some countries -- most notably Russia -- have not implemented government programmes designed to remedy iodine deficiencies in their citizens.

Iodine is an essential element for other animals, too -- most dramatically, amphibians. T3 and T4 regulate genes that control metamorphosis from aquatic water-breathing tadpoles into terrestrial air-breathing adults. Tadpoles that suffer an iodine deficiency remain tadpoles until the end of their days. Interestingly, aquatic salamanders known as mudpuppies, Necturus maculosus, never undergo metamorphosis into the adult form, not because of an environmental iodine deficiency, but because they are resistant to the metamorphic effects of iodine and thyroid hormones [PMID: 16686639].

Here's our favourite chemistry professor telling us more about the wonders of iodine:


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You've already met these elements:

Tellurium: Te, atomic number 52
Antimony: Sb, atomic number 51
Tin: Sn, atomic number 50
Indium: In, atomic number 49
Cadmium: Cd, atomic number 48
Silver: Ag, atomic number 47
Palladium: Pd, atomic number 46
Rhodium: Rh, atomic number 45
Ruthenium: Ru, atomic number 44
Technetium: Tc, atomic number 43
Molybdenum: Mo, atomic number 42
Niobium: Ni, atomic number 41
Zirconium: Zr, atomic number 40
Yttrium: Y, atomic number 39
Strontium: Sr, atomic number 38
Rubidium: Rr, atomic number 37
Krypton: Kr, atomic number 36
Bromine: Br, atomic number 35
Selenium: Se, atomic number 34
Arsenic: As, atomic number 33
Germanium: Ge, atomic number 32
Gallium: Ga, atomic number 31
Zinc: Zn, atomic number 30
Copper: Cu, atomic number 29
Nickel: Ni, atomic number 28
Cobalt: Co, atomic number 27
Iron: Fe, atomic number 26
Manganese: Mn, atomic number 25
Chromium: Cr, atomic number 24
Vanadium: V, atomic number 23
Titanium: Ti, atomic number 22
Scandium: Sc, atomic number 21
Calcium: Ca, atomic number 20
Potassium: K, atomic number 19
Argon: Ar, atomic number 18
Chlorine: Cl, atomic number 17
Sulfur: S, atomic number 16
Phosphorus: P, atomic number 15
Silicon: Si, atomic number 14
Aluminium: Al, atomic number 13
Magnesium: Mg, atomic number 12
Sodium: Na, atomic number 11
Neon: Ne, atomic number 10
Fluorine: F, atomic number 9
Oxygen: O, atomic number 8
Nitrogen: N, atomic number 7
Carbon: C, atomic number 6
Boron: B, atomic number 5
Beryllium: Be, atomic number 4
Lithium: Li, atomic number 3
Helium: He, atomic number 2
Hydrogen: H, atomic number 1



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