Discovered | Characteristics | Found | Prepared | Reactions | Uses

Iodine is a nonmetal in Group VIIA (the halogens, Group 17) and the 5th period. Other elements in the group are fluorine (F), chlorine (Cl), bromine (Br), and astatine (At).

Iodine was discovered in 1811 by the French chemist Bernard Courtois. He and his father were salt merchants, that is, they sold sodium nitrate and other salts of commercial value. One way to obtain sodium and potassium salts was to burn the seaweed and algae cast up on the beaches of France. The ash contains chlorides, bromides, iodides, carbonates, and sulfates of Na, K, Mg, and Ca. Apparently to remove the carbonates Courtois treated the salts leached from the ash with sulfuric acid, and he saw a purple cloud of iodine, I2, rise from the reaction.

The element is a purple-black solid, unlike fluorine and chlorine (both gases) and bromine (a liquid). The solid is volatile, however, and can be sublimed easily. (The vapor pressure of the solid at 90°C is 26.8 mmHg.)

Iodine's chemistry is similar to that of the other halogens. Its common oxidation numbers are -1, 0, +1, +3, +5, and +7. It is a less powerful oxidizing agent than the other halogens, and the others can displace iodine from its salts.

Br2 + 2 KI(aq) 2 KBr(aq) + I2

It forms compounds with all other elements except for S, Se, and the rare gases. It does not react directly with C, N2, and O2, but it does so with almost all other elements.

Iodine is widely distributed in the earth's crust, although it is only 47th in abundance (0.46 ppm). Most of the iodine is recovered from natural brine wells (wells containing aqueous NaCl and other salts) in Oklahoma and Michigan. Wells in Japan are particularly rich in iodides, and that country is the dominant producer.

The pure element is prepared by first precipitating copper(II) iodide from a KI solution.

2 KI(aq) + CuSO4(aq) CuI2(s) + K2SO4(aq)

The copper(II) iodide disproportionates to give copper(I) iodide and I2.

CuI2(s) CuI(s) + ½ I2(g)

Iodine is soluble in a number of organic solvents but only slightly soluble in water.

I2 does not react with water or acids. In basic solution, however, it is like Cl2 and Br2 in that it disproportionates to iodide and hypoiodite.

I2(aq, basic) + 2 OH-(aq) I-(aq) + OI-(aq) + H2O

This is the reason that the color of I2 disappears on adding aqueous NaOH to a CCl4 solution of I2.

The element has important medical uses. Very shortly after its discovery it was realized that its salts can be used to treat goiters (now known to arise from a thyroid disorder), and the table salt sold in the U.S. today contains a trace of KI as a preventive measure. A tincture of iodine - an alcohol solution of the element - is used as a topical antiseptic. A trace of CH3I (methyl iodide) vapor is added to "halogen" headlights.