Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a lustrous, purple-black non-metallic solid at standard conditions that melts to form a deep violet liquid at 114 degrees Celsius, and boils to a violet gas at 184 degrees Celsius.
In 1811 the French chemist Bernard Courtois obtained a violet vapour by heating seaweed ashes with sulfuric acid as a by-product of the manufacture of saltpetre. This vapour condensed to a black crystalline substance, which he called “substance X.” In 1813 British chemist Sir Humphry Davy, who was passing through Paris on his way to Italy, recognized substance X as an element analogous to chlorine; he suggested the name iodine from the Greek word ioeides which means “violet coloured”.
Iodine and its compounds are primarily used in nutrition. Due to its high atomic number and ease of attachment to organic compounds, it has also found favour as a non-toxic radiocontrast material. Because of the specificity of its uptake by the human body, radioactive isotopes of iodine can also be used to treat thyroid cancer. It is also used as a catalyst in the industrial production of acetic acid and some polymers.
Physical And Chemical Properties of Iodine
- It is a nonmetallic, nearly black solid at room temperature and has a glittering crystalline appearance. The molecular lattice contains discrete diatomic molecules, which are also present in the molten and the gaseous states. Above 700 °C (1,300 °F), dissociation into iodine atoms becomes appreciable.
- It has a moderate vapour pressure at room temperature and in an open vessel slowly sublimes to a deep violet vapour that is irritating to the eyes, nose, and throat. Highly concentrated iodine is poisonous and may cause serious damage to skin and tissues.
- Molten I may be used as a nonaqueous solvent for iodides. The electrical conductivity of molten iodine has in part been ascribed to the following self-ionization equilibrium.
- The electron affinity of the iodine atom is not much different from those of the other halogen atoms. It is a weaker oxidizing agent than bromine, chlorine, or fluorine.