Crystalline iodine. (Source: © Remigiusz / stock.adobe.com)

A chemical element with atomic number 53. Iodine is a halogen. At room temperature, it is a semi-lustrous, non-metallic black substance that sublimes gradually into violet vapors.

The only naturally occurring isotope of iodine is stable iodine 127. Radioactive isotopes of iodine with half-lives of about a few days are used for radiodiagnostics and radiotherapy, mostly of thyroid cancer. The radioisotope of iodine ¹³¹I is a typical product of the fission reaction and a compound of spent fuel.