Iodine is a nonmetallic chemical element which is found abundantly in seawater and in some minerals in the earth. It is classified in the halogens, along with elements like chlorine and bromine, and like other elements in this group, iodine is reactive and toxic in large amounts. There are a number of useful applications for iodine, especially in the field of medicine, and the element is also a vital trace element needed for proper human nutrition.

In pure form, iodine is a lustrous substance with a purple to black color. It vaporizes into a lovely blue-purple gas which also happens to be quite toxic. Pure iodine is corrosive and poisonous, and it should be handled very carefully, ideally while wearing gloves and face protection. The element is identified with the symbol I and it has an atomic number of 53. Although slightly less reactive than some other elements in the halogen group, iodine will react readily with a range of substances.

Many people are familiar with iodine in the form of a solution used in the field of medicine as a germicide or antiseptic. Iodine can also be used to produce a purple dye, and its radioactive isotopes are used in medical diagnosis. Radioactive iodine can be used to treat thyroid cancer and as a tracer in the body to determine how well the thyroid is functioning. Humans need around 140 micrograms of iodine a day; this dietary iodine is usually taken in through fish, sea vegetables, and other foods of marine origin. In many parts of the world, iodine is added to salt to ensure that people get enough dietary iodine; this product is known as iodized salt.

The element was discovered in 1811 by Bernard Courtois, who was working with seawater to extract sodium carbonate. He did not measure his chemicals correctly, and he ended up producing a purple vapor which quickly crystallized into iodine. Several chemists including Humphry Davy confirmed the discovery; Davy at one point attempted to take credit for finding iodine on his own. Most iodine today is extracted from caliche, a mineral compound which contains iodine, rather than seawater.

Pure iodine is toxic. It should not be ingested or handled with bare hands, and the vapor is very irritating so it should be handled in a well ventilated area. Since iodine is so volatile, it is important to be careful about handling it with other chemicals. Iodine solutions such as those used to disinfect surgical patients are safe to handle although they should not be ingested.