Chemical Element: iodine

(Greek: iodes, "violet"; from the color of its vapor; nonmetal)

Chemical-Element Information

Symbol: I
Atomic number: 53
Year discovered: 1811

Discovered by: Bernard Courtois (1777-1838), a French chemist.


  • Courtois was in the business of manufacturing potassium nitrate (needed in the making of gunpowder).
  • He got it from potassium carbonate (potash), which in turn he got from seaweed.
  • He isolated iodine from treating seaweed ash with sulphuric acid while recovering sodium and potassium compounds.
  • One day, in 1811, he added too much acid and, on heating, he obtained a beautiful violet vapor.
  • After condensing the vapor, he found dark, lustrous crystals.
  • Iodine is a bluish-black, lustrous solid.
  • Courtois suspected it might be a new element and he passed it on to other chemists for confirmation.
  • It was a new element, and Humphry Davy suggested that it be named “iodine”, from the Greek word for “violet”.
  • It volatilizes at ambient temperatures into a pretty blue-violet gas with an irritating odor.
  • It forms compounds with most elements, but is less reactive than the other halogens, which displace it from iodides.
  • Iodine exhibits some metallic-like properties. It dissolves readily in chloroform, carbon tetrachloride, or carbon disulphide to form beautiful purple solutions.
  • It is only slightly soluble in water.
  • Iodine compounds are important in organic chemistry and very useful in medicine and photography.
  • In humans, not having enough iodine often results in goiter.
  • The deep blue color with starch solution is characteristic of the free element of iodine.
  • It is assimilated by seaweeds from which it may be recovered, and is found in Chilean saltpetre, caliche, old salt brines, and salt wells.
  • Iodine is available commercially so it is not normally necessary to make it in the laboratory.
  • Iodine occurs in seawater but in much smaller quantities than chloride or bromide.
  • With suitable sources of brine, it is recovered commercially through the treatment of brine with chlorine gas and flushing through with air.
  • In this treatment, iodide is oxidized to iodine by the chlorine gas.

Name in other languages:

French: iode

German: Iod

Italian: iodio

Spanish: yodo


Information about other elements may be seen at this Chemical Elements List.

A special unit about words that include chemo-, chem- may be seen here.