I Iodine

Iodine is mainly used in iodised table salt, which has nearly wiped out iodine deficiency in much of the world. Hospitals use it to disinfect skin, to image organs in CT scans, and to treat overactive thyroids. Its compounds also go into photography, dyes and medicines, and it is obtained from seaweed, brine wells and Chilean nitrate deposits.

The chemical symbol for iodine is I, from its name, which comes from the Greek word ioeides, meaning violet-coloured, after the violet vapour it gives off when heated. It has atomic number 53 and a standard atomic mass of 126.9, and Bernard Courtois discovered it in 1811 in the ash of seaweed.

Iodine has atomic number 53, so it is the fifty-third element in the periodic table. Its atom holds fifty-three protons and fifty-three electrons, and its standard atomic mass is 126.9. It is a halogen in group 17 and period 5, with the electron configuration [Kr] 4d10 5s2 5p5, and it was discovered in 1811.

No, iodine is a nonmetal. It is a halogen in group 17 of the periodic table, and at room temperature it is a shiny purple-black solid. It sublimes when warmed, turning directly into a violet gas without melting first. It is the heaviest halogen that is essential to life, because the thyroid needs it to make hormones.

Iodine is added to table salt because many soils far from the sea contain too little iodine, and without it the thyroid cannot make hormones, causing goitre and stunted growth in children. Iodised salt is a cheap way to give everyone the small amount they need, and it has nearly wiped out iodine deficiency in much of the world.