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126.90447(3) 0

1s22s22p63s23p63d104s24p6 4d105s25p5 5,7
[ ? ] Halogen:

Iodine, chemical symbol I, is a chemical element with atomic number 53 that is typically a solid in its elemental form but is easily sublimed into a gas. It consists of both stable isotopes and radioactive isotopes. Radioactive iodine has been released in past disasters at nuclear energy facilities.

Iodine has an atomic mass of 126.90447 g•mol −1, melts at 113.7 °C and boils at 184.4 °C. It is a halogen and, thus, is often found in salts as I−1 in which the gain of one electron fulfills the octet rule to form a very stable valence shell.

Chemical applications

Iodine is specified as a List I chemical by the US Drug Enforcement Administration and is considered to have high risk of diversion to illicit drug manufacturing.[1]

Sublimed iodine has been used as a general reagent for developing invisible ink, as it recrystallizes on paper fibers disturbed by the act of writing. For this purpose, however, it has largely been displaced by specialized photographic methods.

Radioactive iodine

Several isotopes of iodine, including 123I, 124I, 129I and 131I are radioactive isotopes of iodine used to treat certain cancers or for medical imaging processes.[2][3]

Both 129I and 131I have been produced from: [4]

  1. Nuclear weapons production and atmospheric testing
  2. Deliberate production for medical use, involving the irradiation of tellurium
  3. Nuclear fission in nuclear energy facilities, and released through accidents such as Three Mile Island.

For reasons of national security, the knowledge of contamination was suppressed in the United States.[4]

The release of iodine-131 in a serious power reactor accident could be retarded by absorption on metal surfaces within the nuclear plant. [5]

  • Glänneskog H (2005) Iodine chemistry under severe accident conditions in a nuclear power reactor, PhD thesis, Chalmers University of Technology, Sweden
  • For other work on the iodine chemistry which would occur during a bad accident, see [1]

Decay and decay products

131I has a half-life of only 8.06 days[6], so environmental spills are of much less concern that similar spills of other radioactive elements such as caesium or uranium.

However, 129I has a half-life of 15.7 million years.[7] Thus, a spill of 131I one thousand times the normal background would decay back to normal levels in about 81 days. The 129I and 131I atoms emit beta particles and emit gamma radiation during radioactive decay. Because iodine is readily absorbed by the thyroid gland, which uses it to produce thyroid hormones, ingestion of radioactive iodine can lead to thyroid pathology, including thyroid cancer.


Like bromine, iodine is readily sublimed, going from the solid state directly to the gaseous state,(skipping the liquid state) so exposure to the solid form can still lead to inhalation of the chemical. Being a halogen, it also readily forms many salts which are readily soluble, so the molecular form is quickly converted into various salts upon reaction with most environments. Dairy animals exposed to any form of radioactive iodine can thus lead to ingestion by humans.

Acute chemical toxicity

Competitive inhibition of radioactive iodine