10 Iodine Facts (Atomic Number 53 or I)

Facts about the Element Iodine

Iodine is a violet vapor or blue-black solid.
Iodine is a violet vapor or blue-black solid. Matt Meadows/Getty Images

Iodine is element 53 on the periodic table, with element symbol I. Iodine is an element you encounter in iodized salt and some dyes. A small amount of iodine is essential for nutrition, while too much is toxic. Here are facts about this interesting, colorful element.

The Name

Iodine comes from the Greek word iodes, which means violet. Iodine vapor is violet-colored. The element was discovered in 1811 by French chemist Bernard Courtois. Courtois discovered iodine by accident while he was making saltpeter for use in the Napoleonic Wars. Making saltpeter required sodium carbonate. To get sodium carbonate, Courtois burned seaweed, washed the ash with water, and added sulfuric acid to remove contaminants. Courtois discovered adding an excess of sulfuric acid produced a cloud of purple vapor. While Courtois believed the vapor was a previously unknown element, he couldn't afford to research it, so he offered samples of the gas to his friends, Charles Bernard Desormes and Nicolas Clement. They characterized the new material and made Courtois' discovery public.


Many isotopes of iodine are known. All of them are radioactive except for I-127, which is the only isotope found in nature. Because there is only one natural isotope of iodine, its atomic weight is precisely known, rather than an average of isotopes like most elements.

Color and Other Properties

Solid iodine is blue-black in color, with a metallic sheen. At ordinary temperatures and pressures, iodine sublimates into its violet gas, so the liquid form is not seen. The color of iodine follows a trend seen in the halogens: they appear progressively darker as you move down the group of the periodic table. This trend happens because the wavelengths of light absorbed by the elements increases due to the behavior of the electrons. Iodine is slightly soluble in water and more soluble in nonpolar solvents. Its melting point and boiling point are the highest of the halogens. The bond between atoms in the diatomic molecule is the weakest in the element group.


Iodine is a halogen, which is a type of non-metal. It is located beneath fluorine, chlorine, and bromine on the periodic table, making it the heaviest stable element in the halogen group.


The thyroid gland uses iodine to make the hormones thyroxine and triiodotyronine. Insufficient iodine leads to development of a goiter, which is a swelling of the thyroid gland. Iodine deficiency is believed to be the leading preventable cause of mental retardation. Excessive iodine symptoms are similar to those of iodine insufficiency. Iodine toxicity is more severe if a person has a selenium deficiency.


Iodine occurs in compounds and as the diatomic molecule I2.

Medical Purpose

Iodine is used extensively in medicine. However, some people develop a chemical sensitivity to iodine. Sensitive individuals may develop a rash when swabbed with tincture of iodine. In rare cases, anaphylactic shock has resulted from medical exposure to iodine.

Food Source

Natural food sources of iodine are seafood, kelp and plants grown in iodine-rich soil. Potassium iodide often is added to table salt to produce iodized salt.

Atomic Number

The atomic number of iodine is 53, meaning all atoms of iodine possess 53 protons.

Commercial Source

Commercially, iodine is mined in Chile and extracted from iodine-rich brine, notably from the oilfields in the US and Japan. Prior to this, iodine was extracted from kelp.

Iodine Element Fast Facts

  • Element Name: Iodine
  • Element Symbol: I
  • Atomic Number: 53
  • Atomic Weight: 126.904
  • Group: Group 17 (Halogens)
  • Period: Period 5
  • Appearance: Metallic blue-black solid; violet gas
  • Electron Configuration: [Kr] 4d10 5s2 5p5
  • Melting Point: 386.85 K ​(113.7 °C, ​236.66 °F)
  • Boiling Point: 457.4 K ​(184.3 °C, ​363.7 °F)


  • Davy, Humphry (1 January 1814). "Some Experiments and Observations on a New Substance Which Becomes a Violet Coloured Gas by Heat". Phil. Trans. R. Soc. Lond. 104: 74. doi:10.1098/rstl.1814.0007
  • Emsley, John (2001). Nature's Building Blocks (Hardcover, First ed.). Oxford University Press. pp. 244–250. ISBN 0-19-850340-7.
  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
  • Swain, Patricia A. (2005). "Bernard Courtois (1777–1838) famed for discovering iodine (1811), and his life in Paris from 1798" (PDF). Bulletin for the History of Chemistry. 30 (2): 103.
  • Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 0-8493-0464-4.