Iodine (I)


  • Symbol: I
  • Atomic Number: 53
  • Atomic Weight: 126.90447
  • Element Classification: Halogen
  • Discovered By: Bernard Courtois
  • Discovery Date: 1811
  • Name Origin: Greek: ‘iodes’ (violet), referring to the violet vapor it produces when heated
  • Density(g/cc): 4.93 (at 20°C)
  • Melting Point: 113.7°C
  • Boiling Point: 184.3°C
  • Appearance: Shiny, black-gray solid; violet vapor
  • Atomic Radius(pm): 133


Iodine was discovered in 1811 by the French chemist Bernard Courtois. He produced iodine accidentally while extracting potassium nitrate (saltpeter) from seaweed ash, a component used in gunpowder manufacturing during the Napoleonic Wars. Upon adding sulfuric acid to seaweed ash to increase the potassium nitrate yield, Courtois observed a violet vapor that condensed into dark crystals. This substance was iodine, and its discovery was later investigated and confirmed by other chemists, including Joseph Louis Gay-Lussac and Humphry Davy.

Relation to Other Elements

Iodine is a member of the halogen group (group 17) in the periodic table, alongside fluorine, chlorine, bromine, and astatine. Halogens are characterized by their high reactivity and tendency to form salts with metals. Iodine is the heaviest stable halogen and shares similar chemical properties with its lighter counterparts, such as forming diatomic molecules (I₂) and participating in halogen bonding. It has a relatively lower reactivity compared to chlorine and bromine.

Natural Occurrence

Iodine is relatively rare in the Earth’s crust but is more abundant in seawater and certain mineral deposits. It is commonly found in the form of iodide (I-) in seawater, underground brines, and in mineral deposits in Chile and Japan. The natural production of iodine is also a result of biological processes, with marine algae and kelp being significant contributors to the iodine cycle.


Iodine has various important applications:

  • Medicine: Iodine is crucial for human health, especially for the synthesis of thyroid hormones, which regulate metabolism. Iodized salt is the primary source of dietary iodine to prevent iodine deficiency disorders.
  • Disinfectants: Iodine and its compounds, such as povidone-iodine, are used as antiseptics and disinfectants in medical settings and water treatment.
  • Radiopharmaceuticals: Radioactive isotopes of iodine, particularly iodine-131, are used in nuclear medicine for diagnosing and treating thyroid conditions, including thyroid cancer.
  • Photography: Silver iodide (AgI) was historically used in photography for its light-sensitive properties.
  • Chemical Synthesis: Iodine is used as a catalyst and reactant in the synthesis of certain organic compounds, including pharmaceuticals and agricultural chemicals.

The discovery of iodine not only filled a gap in the chemistry of halogens but also had a profound impact on medicine, nutrition, and various industrial processes. Its role in thyroid health and widespread use in antiseptics highlight the critical importance of this element to human well-being and technology.

Tellurium (Te)

Xenon (Xe)