Iodine

Iodine (chemical symbol: I) is a chemical element with atomic number 53. It belongs to the halogen group, which also includes fluorine, chlorine, and bromine. In its common state, it is a hard, dark-grey solid with a lustrous sheen; when heated rapidly, it transforms into a blue-grey vapour.

Iodine occurs in small quantities in rocks and soil, but is more abundant in the sea and in marine organisms such as fish and seaweed. It is the only halogen directly required for normal human body function.

Chemical Properties

Iodine is the least reactive of the halogens in common use. It has lower chemical reactivity than chlorine and bromine, making it relatively unreactive with other elements. It dissolves poorly in water, but dissolves readily in alcohol and in potassium iodide solutions, producing a brown-black colour.

Role in the Body

The thyroid gland uses iodine to produce the hormones thyroxine and triiodothyronine. These hormones regulate metabolic rate, growth, and brain development, especially during pregnancy and early childhood.

Iodine deficiency causes thyroid enlargement, known as goiter, and impaired thyroid function. Children whose mothers were iodine-deficient during pregnancy may develop developmental problems. For this reason, many countries have adopted policies to add iodine to table salt (iodized salt).

Uses

Iodine solutions such as tincture of iodine and povidone-iodine are used to disinfect wounds and skin before surgery, because iodine kills bacteria. In industry, iodine is used in conventional photography, pharmaceutical manufacture, and laboratory analysis; for example, iodine produces a blue-black colour when added to starch, a well-known test.

The radioactive form iodine-131 is used to treat and diagnose thyroid disease. When a nuclear reactor accident occurs, potassium iodide tablets are distributed so that the thyroid becomes saturated with normal iodine and does not take up the radioactive form.