Iodine is a slate-gray, crystalline, nonmetallic substance belonging to the halogen group of elements. Halogens — which include chlorine, bromine and fluorine — are highly reactive elements, so iodine always is used as a compound with another substance such as a metal. When heated, iodine crystals vaporize, or sublimate, into a violet-colored gas. Iodine occurs in trace amounts in saltpeter and sodium nitrate and as ions in the oceans. Iodine is essential to many life-forms, and most living organisms contain trace amounts of iodine. Kelp, oysters and crustaceans absorb iodine from seawater.

Silver iodide is the principal light-sensitive substance in photographic films, papers and plates. The surface is covered with a suspension of silver iodide grains. This substance reacts with light to form black silver atoms. These atoms, in turn, deposit on the film, paper or plate to form an image.

Silver iodide has been used by meteorologists in cloud seeding, a method of modifying the weather. Its crystalline structure is the same as ice's. The crystals serve as nuclei around which water can condense and increase precipitation.

Polarizers are used in many optical instruments and displays. Iodine-based films have superior optical qualities to dye-based films. They are used as contrast enhancers in liquid crystal displays (LCD).

Iodine is one element that can act as a radioactive tracer, a substance with a radioactive isotope that emits radiation as it passes through a medium. A receiver tracks the progress of the isotope. In medical diagnostics, iodine is used in X-ray, ultrasound and nuclear imaging scans such as computerized axial tomography, or CAT, scans. Health care is the largest market for iodine. Tracers injected into a complex industrial plant such as an oil refinery can track faults in machinery and leaks.

The U.S. Environmental Protection Agency issued an approval for the use of methyl iodide as an agricultural pesticide in 2007. It is used to fumigate soil before planting. Methyl iodide replaced methyl bromide, as the latter damaged the ozone layer. Scientists are concerned that methyl iodide has carcinogenic effects.