Iodine is a chemical element that has the symbol I and the atomic number 53.

Chemically, iodine is the second least reactive of the halogens, and the second most electropositive halogen, trailing astatine in both aspects. However, the element does not occur in the free state in nature. As with all other halogens (members of Group 17 in the periodic table), when freed from its compounds iodine forms diatomic molecules (I2).

Iodine and its compounds are primarily used in medicine, photography, and dyes.


Iodine under standard conditions is a bluish black solid. It can be seen apparently sublimating at standard temperatures into a violet-pink gas that has an irritating odour. This halogen forms compounds with many elements, but is less reactive than the other members of its Group VII (halogens) and has some metallic light reflectance. In its gas phase iodine shows its violet color. Elemental iodine dissolves easily in most organic solvents such as hexane or chloroform due to its lack of polarity, but is only slightly soluble in water. However, the solubility of elemental iodine in water can be increased by the addition of potassium iodide. The molecular iodine reacts reversibly with the negative ion, generating the triiodide anion I3− in equilibrium, which is soluble in water. This is also the formulation of some types of medicinal (antiseptic) iodine, although tincture of iodine classically dissolves the element in aqueous ethanol. Solutions of elemental iodine have the unique property of exhibiting dramatically different colors depending on the polarity of the solvent. When dissolved in nonpolar solvents like hexane, the solution appears deep violet; in moderately polar dichloromethane the solution is dark crimson, and in strongly polar solvents like acetone or ethanol, it appears dark orange or brown. This is due to ligand field interactions of solvent molecules with the d-orbitals of iodine, which is the only halogen with a sufficiently occupied electronic configuration to allow such interactions. This same property allows the formation of hypervalent iodine compounds, which have expanded bonding orbitals beyond the generally allowed octet rule.

Students who have seen the classroom demonstration in which iodine crystals are gently heated in a test tube to violet vapour may gain the impression that liquid iodine does not exist at atmospheric pressure. This misconception arises because the vapour produced has such a deep colour that the liquid appears not to form. In fact, if iodine crystals are heated carefully to just above their melting point of 113.7 °C, the crystals melt into a liquid which is present under a dense blanket of the vapour.


The base value of each unit of ranges between 1 and 20Ð per unit, with up to 4 units being found at any one time.

Presence on Mars: Common

Martian Minerals
Group 1 | Group 2 | Group 3 | Group 4 | Group 5 | Group 6
Group 3 |Antimony | Astatine | Barium | Bismuth | Cesium | Francium | Hafnium | Indium | Iodine | Iridium | Lanthanum | Lead | Mercury | |Osmium | Platinum | Polonium | Radium | Radon | Rhenium | Tantalum | Tellurium | Thallium | Tin | Tungsten | Xenon|

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