A flask containing iodine. Iodine is a lustrous metallic grey crystalline solid. At room temperature it partly sublimates to form a bright purple vapour. It has an atomic number of 53, the symbol I and is a member of the halogens (group 17). In its elemental form it is diatomic with the formula I2. Its compounds are used as disinfectants, as a radio-contrast agent in X-rays and in seeding rainclouds.