Iodine. Crystals of the element iodine (chemical symbol: I) heated to produce iodine vapour. Iodine is a violet-black non-metallic element which gives off a violet vapour. With a melting point of 113. 5 degrees centigrade, it belongs to the halogen group of elements on the periodic table. Iodine is found in seawater and was formerly extracted from seaweed; it is now obtained from oil-well brines. Iodine is required as a trace element by most living organisms. In mammals it is concentrated in the thyroid gland as a constituent of thyroid hormones. Iodine was discovered by Courtois in 1812.