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==Properties==
[[File:IodoAtomico.JPG|thumb|left|upright=0.7|alt=Round bottom flask filled with violet iodine vapour|Iodine vapour in a flask, demonstrating its characteristic rich purple colour]]
Iodine is the fourth [[halogen]], being a member of group 17 in the periodic table, below [[fluorine]], [[chlorine]], and [[bromine]]; since [[astatine]] and [[tennessine]] are radioactive, iodine is the heaviest stable halogen. Iodine has an electron configuration of [Kr]4d5s<sup>102</sup>5s4d<sup>210</sup>5p<sup>5</sup>, with the seven electrons in the fifth and outermost shell being its [[valence electron]]s. Like the other halogens, it is one electron short of a full octet and is hence an oxidising agent, reacting with many elements in order to complete its outer shell, although in keeping with [[periodic trends]], it is the weakest oxidising agent among the stable halogens: it has the lowest [[electronegativity]] among them, just 2.66 on the Pauling scale (compare fluorine, chlorine, and bromine at 3.98, 3.16, and 2.96 respectively; astatine continues the trend with an electronegativity of 2.2). Elemental iodine hence forms [[diatomic molecule]]s with chemical formula I<sub>2</sub>, where two iodine atoms share a pair of electrons in order to each achieve a stable octet for themselves; at high temperatures, these diatomic molecules reversibly dissociate a pair of iodine atoms. Similarly, the iodide anion, I<sup>−</sup>, is the strongest reducing agent among the stable halogens, being the most easily oxidised back to diatomic I<sub>2</sub>.<ref name="Greenwood800">Greenwood and Earnshaw, pp. 800–4</ref> (Astatine goes further, being indeed unstable as At<sup>−</sup> and readily oxidised to At<sup>0</sup> or At<sup>+</sup>.)<ref>{{cite book | series = Gmelin Handbook of Inorganic and Organometallic Chemistry | title = 'At, Astatine', System No. 8a | edition=8th | year = 1985 | publisher = Springer-Verlag | isbn = 978-3-540-93516-2 | vauthors = Kugler HK, Keller C | volume = 8 }}</ref>
The halogens darken in colour as the group is descended: fluorine is a very pale yellow, chlorine is greenish-yellow, bromine is reddish-brown, and iodine is violet.
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