Iodine
Symbol: I
Atomic Number: 53
Atomic Mass: 126.90447 amu (atomic mass units)
Melting Point: 113.5 C (386.65 K, 236.3 F)
Boiling Point: 184.0 C (457.15 K, 363.2 F)
Number of Protons: 53
Number of Electrons: 53
Number of Neutrons: 74
Classification: Non-metal
Group Name: Halogen
Crystal Structure: Orthorhombic
Density @ 293 K: 4.93 grams per cubic centimeter
Color: Blackish
The element iodine was discovered by a laboratory accident. In 1811, the French chemist Barnard Courtois had been extracting sodium and potassium compounds from a sample of seaweed ash. With all these compounds removed, he added sulfuric acid to the remaining ash. He accidently added too much acid, which caused a violet colored cloud of iodine gas to appear. This gas condensed on the metal objects in his laboratory coating them with iodine crystals. The name iodine comes from the Greek word for the color violet “iodes”.
Iodine is an essential trace element in the human body. It is an important constituent of thyroxin, a hormone produced by the thyroid gland. This hormone controls the rate of the body’s mental and physical development. Iodine deficiency can cause a swelling of the thyroid gland known as goiter. Trace amounts of iodine are often added to common table salt (iodized salt) to help prevent iodine deficiency. While essential in trace amounts pure iodine is poisonous if ingested. Iodine can also cause burns to the skin, eyes and mucous membranes, so the pure element must be handled with caution.
The only naturally occurring isotope of iodine is iodine-127, which is stable. There are number of unstable isotopes of iodine with mass numbers ranging from 108 to 144. Iodine-131 is a valuable treatment for some thyroid gland diseases. This isotope has a half-life of 8.02 days and decays by beta decay.
Iodine used to be produced from seaweed but now commercial production of the element is from deposits of sodium iodate and sodium periodate found in Chile and Bolivia. There are a number of uses for iodine and its compound potassium iodide.
* The rapid test for starch commonly used in laboratories utilizes iodine. Iodine produces a blue color when in contact with starch.
* An aqueous solution of iodine and potassium iodide, called Lugols Iodine, is an important step in the Gram stain method used in bacteriology laboratories.
* The medical profession use iodine and potassium iodide in an alcohol-based solution as a topical skin disinfectant and surface wound cleanser.
* Potassium iodide is used in the production of photographic film.
