Iodine is a dark, lustrous halogen. At standard conditions it is a violet-black solid that readily sublimes to a purple vapor. It forms iodide (I−) and high-oxidation oxyanions.
The ground-state configuration is [Kr] 4d10 5s2 5p5. With seven valence electrons (\(5s^2 5p^5\)), iodine behaves like a typical halogen, readily gaining an electron to form iodide, \(\mathrm{I^-}\).
Molecular iodine is composed of discrete \(\mathrm{I_2}\) molecules held together in the solid by relatively weak London dispersion forces compared with the thermal energy available at ambient temperatures. Hence it readily sublimes to a purple vapor:
\(\mathrm{I_2(s) \rightleftharpoons I_2(g)}\)
Iodine spans multiple oxidation states: −1 (iodide), +1 (hypoiodite), +5 (iodate), and +7 (periodate). Important oxyanions include:
In alkaline media, iodine disproportionates:
\(\mathrm{3\,I_2 + 6\,OH^- \rightarrow IO_3^- + 5\,I^- + 3\,H_2O}\)
In the presence of excess iodide, iodine forms triiodide, \(\mathrm{I_3^-}\):
\(\mathrm{I_2 + I^- \rightleftharpoons I_3^-}\)
\(\mathrm{I_3^-}\) intercalates into the helical amylose component of starch, producing a deep blue charge-transfer complex—basis of the classic starch test.
Two major sources are iodide-rich brines and copper anode slimes/caliche. From brine, iodide is oxidized with chlorine:
\(\mathrm{2\,I^- + Cl_2 \rightarrow I_2 + 2\,Cl^-}\)
The \(\mathrm{I_2}\) is then extracted and purified by sublimation or solvent methods.
Lugol's iodine is an aqueous solution containing \(\mathrm{I_2}\) and \(\mathrm{KI}\) (to form soluble \(\mathrm{I_3^-}\)). Tincture of iodine is \(\mathrm{I_2}\) dissolved in ethanol/water, often with \(\mathrm{KI}\). Both are used as antiseptics/iodine supplements (per local guidelines).
Iodine is required for thyroid hormones T3 (triiodothyronine) and T4 (thyroxine). Iodized salt prevents deficiency disorders such as goiter and hypothyroidism. Dietary iodine is mainly present as iodide, \(\mathrm{I^-}\).
In iodometry, analytes generate \(\mathrm{I_2}\) quantitatively, which is then titrated with thiosulfate:
\(\mathrm{I_2 + 2\,S_2O_3^{2-} \rightarrow 2\,I^- + S_4O_6^{2-}}\)
Starch is used near the endpoint; the blue \(\mathrm{I_3^-}\)–starch color disappears at completion.
Iodine forms several interhalogens with other halogens, e.g., \(\mathrm{ICl}\), \(\mathrm{IBr}\), \(\mathrm{IF_5}\), and \(\mathrm{IF_7}\). Fluorides are strong fluorinating agents and place iodine in high oxidation states (+5, +7).
A classic kinetic demonstration where iodine is generated at a controlled rate and suddenly forms a blue complex with starch after a well-defined induction time. One variant involves peroxydisulfate oxidizing iodide to iodine while thiosulfate temporarily reduces it back to iodide until depleted.
Solid/vapor iodine is an eye and respiratory irritant and can stain skin. Some inorganic and organic iodine compounds may be harmful if ingested/inhaled. Use gloves, goggles, and adequate ventilation; store away from reducers and organics as appropriate. Follow local safety regulations.