Iodine was discovered in 1811 by accident along the western coast of France. It was found during
a recovery procedure for sodium and potassium compounds from
seaweeds that were washed ashore at high tides.
Iodine is a solid nonmetal. In its pure state, it is
a black solid. The crystals are easily sublimed
into a violet vapour when heated. Iodine vapour has a pungent odour, is poisonous and is intensely
irritating to the eyes, nose and throat.
Iodine occurs as the iodide ion, I-, in some mineral waters associated with oil wells. The iodide content of the sea water is very low, but it is concentrated by some sea-weeds and sponges. Another source of iodine is sodium iodate, NaIO3, which occurs as a minor constituent in "Chile saltpetre", NaNO3, which is found in dry areas in Chile.
Iodine is used to treat cuts and scrapes on the skin. That substance is a tincture of iodine - a dilute mixture of alcohol and iodine. The human thyroid gland secretes an iodine-bearing hormone called thyroxin. The body needs iodine as an iodine deficiency causes thyroid trouble. Most table salt is now "iodized" to supplement the human diet. As well, iodine is found in seafood.
Radioactive iodine-131, 131I, is used in medical diagnosis to monitor and trace the flow of thyroxin from the thyroid gland.