Periodic Table Element Facts: Iodine

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Iodine Basic Facts

Atomic Number: 53

Iodine Symbol: I

Atomic Weight: 126.90447

Discovery: Bernard Courtois 1811 (France)

Electron Configuration: [Kr] 4d10 5s2 5p5

Word Origin: Greek iodes, violet

Isotopes: Twenty-three isotopes of iodine are known. Only one stable isotope is found in nature, I-127.


Iodine has a melting point of 113.5°C, a boiling point of 184.35°C, a specific gravity of 4.93 for its solid state at 20°C, a gas density of 11.27 g/l, with a valence of 1, 3, 5, or 7. Iodine is a lustrous blue-black solid which volatizes at room temperature into a violet-blue gas with an irritating odor. Iodine forms compounds with many elements, but it is less reactive than the other halogens, which will displace it. Iodine also possesses some properties typical of metals. Iodine is only slightly soluble in water, although it dissolves readily in carbon tetrachloride, chloroform, and carbon disulfide, forming purple solutions. Iodine will bind to starch and color it deep blue. Although iodine is essential for proper nutrition, care is needed when handling the element, as skin contact can cause lesions and the vapor is highly irritating to the eyes and mucous membranes.


The radioisotope I-131, with a half-life of 8 days, has been used to treat thyroid disorders. Insufficient dietary iodine leads to the formation of a goiter. A solution of iodine and KI in alcohol is used to disinfect external wounds. Potassium iodide is used in photography and radiation pills.


Iodine is found in the form of iodides in seawater and in the seaweeds which absorb the compounds. The element is found in Chilean saltpeter, and nitrate-bearing earth (caliche), brackish waters from salt wells and oil wells, and in brines from old sea deposits. Ultrapure iodine may be prepared by reacting potassium iodide with copper sulfate.

Element Classification: Halogen

Iodine Physical Data

Density (g/cc): 4.93

Melting Point (K): 386.7

Boiling Point (K): 457.5

Appearance: shiny, black nonmetallic solid

Atomic Volume (cc/mol): 25.7

Covalent Radius (pm): 133

Ionic Radius: 50 (+7e) 220 (-1e)

Specific Heat (@20°C J/g mol): 0.427 (I-I)

Fusion Heat (kJ/mol): 15.52 (I-I)

Evaporation Heat (kJ/mol): 41.95 (I-I)

Pauling Negativity Number: 2.66

First Ionizing Energy (kJ/mol): 1008.3

Oxidation States: 7, 5, 1, -1

Lattice Structure: Orthorhombic

Lattice Constant (Å): 7.720

References: Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange's Handbook of Chemistry (1952), CRC Handbook of Chemistry & Physics (18th Ed.)

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Your Citation
Helmenstine, Anne Marie, Ph.D. "Periodic Table Element Facts: Iodine." ThoughtCo, Jul. 29, 2021, Helmenstine, Anne Marie, Ph.D. (2021, July 29). Periodic Table Element Facts: Iodine. Retrieved from Helmenstine, Anne Marie, Ph.D. "Periodic Table Element Facts: Iodine." ThoughtCo. (accessed June 5, 2023).