Last revised by Daniel J Bell on 27 Sep 2021

Iodine (chemical symbol I) is one of the trace elements. Its biological importance is its central place in the physiology of the thyroid gland and, in radiology, as the key chemical constituent of most of the radiographic, fluoroscopic, and CT contrast media.

Iodine has the atomic number 53 with an atomic weight of 126.90 Da. It is a shiny purplish solid in the halogen group with an unusual odor and unpleasant taste. It has an approximate k-shell binding energy of 33.2 keV.

All natural iodine on earth is found in the form of the stable isotope iodine-127 3. In addition almost another 40 radioactive isotopes of iodine are known to exist, the most stable being iodine-129, with a half-life of  1.7 × 107 years 3

  • iodine is of vital importance as one of the elements in thyroxine (tetraiodothyronine, T4) and thyronine (tri-iodothyronine, T3), the thyroid hormones
  • iodine is the key constituent element in the majority of contrast media used for CT and fluoroscopy
  • iodine isotopes have been widely employed as radiotracers in nuclear medicine, both as diagnostic and therapeutic agents

Elemental iodine was discovered in 1811 by Bernard Courtois, a French chemist, as a gas produced when seaweed was doused with sulfuric acid (during synthesis of nitre, a precursor to the manufacture of fertilisers and gunpowder). It was another two years before Louis Gay-Lussac and Sir Humphry Davy crystallized the gas as a purplish solid. They named it for its distinctive color, from the Ancient Greek word ιωδης, (iodes) meaning violet-colored 1.

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Cases and figures

  • Figure 1: iodine
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  • Figure 2: periodic table
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