IODINE SYMBOL:฀I฀ PERIOD:฀5฀ GROUP:฀17฀ ATOMIC฀NO:฀53

IODINE SYMBOL:฀I฀ PERIOD:฀5฀ GROUP:฀17฀ ATOMIC฀NO:฀53

ATOMIC฀MASS:฀126.9044฀amu฀ VALENCE:฀1,฀3,฀5,฀and฀7฀ OXIDATION฀STATE:฀+1,฀–1,฀ +5,฀and฀+7฀ NATURAL฀STATE:฀Solid ORIGIN฀OF฀NAME:฀The฀name฀originates฀from฀the฀Greek฀word฀iodes,฀meaning฀“violet-col- ored,”฀which฀is฀the฀color฀of฀iodine’s฀vapor. ISOTOPES:฀There฀are฀a฀total฀of฀145฀isotopes฀of฀iodine.฀Only฀one฀(I-127)฀is฀stable฀and฀ accounts฀for฀100%฀of฀iodine’s฀natural฀abundance฀on฀Earth.฀All฀the฀other฀146฀isotopes฀ are฀radioactive฀with฀half-lives฀ranging฀from฀a฀150฀nanoseconds฀to฀1.57×10 +7 ฀years.

ELECTRON฀CONFIGURATION ฀ Energy฀Levels/Shells/Electrons฀ Orbitals/Electrons

s2,฀p6

฀ 3-M฀=฀18฀

s2,฀p6,฀d10

฀ 4-N฀=฀18฀

s2,฀p6,฀d10

฀ 5-O฀=฀7฀

s2,฀p5

255 Properties

Guide to the Elements |

Iodine in its pure state is a black solid that sublimates (changes from a solid to a gas without going through a liquid state) at room temperature. It produces a deep purple vapor that is irri- tating to the eyes, nose, and throat. Iodine tends to form nonmetallic diatomic molecules (I 2 ). It is the heaviest of the naturally occurring halogens. (Although astatine, the fifth element in group 17, is heavier than iodine, it is a synthetic element and does not occur in nature except as a very small trace.) Iodine is the least reactive of the five halogens.

Iodine’s melting point is 113.7°C, its boiling point is 184.4°C, and its density is 4.93g/ cm 3 .

Characteristics Iodine is the least reactive of the elements in the halogen group 17. Most people associate

iodine with the dark-brown color of the tincture of iodine used as an antiseptic for minor skin abrasions and cuts. A tincture is a 50% solution of iodine in alcohol. Although it is still used, iodine is no longer the antibiotic of choice for small skin wounds. Since iodine is a poison that kills bacteria, iodine tablets are often used by campers and others to purify water that is taken from outdoor streams.

Abundance฀and฀Source Iodine is the 64th most abundant element on Earth. It occurs widely over the Earth, but

never in the elemental form and never in high concentrations. It occurs in seawater where some species of seaweed and kelp accumulate the element in their cells. It is also recovered from deep brine wells found in Chile, Indonesia, Japan, and Michigan, Arkansas, and Oklahoma in the United States. The iodine is recovered from cremated ashes of seaweed. The ashes are leached with water to remove the unwanted salts.

Finally, manganese dioxide (MnO 2 ) is added to oxidize the iodine ions (I 1- ) to produce elemental diatomic iodine (I

2 ). The following reaction takes place: 4I + MnO 2 → MnI 2 +

I + 2O 2 2- . Chilean saltpeter [potassium nitrate (KNO 3 )] has a number of impurities, including sodium and calcium iodate. Iodine is separated from the impurities and, after being treated chemically, finally produces diatomic iodine. Today, iodine is mostly recovered from sodium

iodate (NaIO 3 ) and sodium periodate (NaIO 4 ) obtained from Chile and Bolivia. History Similar to the history of many other elements, iodine’s discovery was serendipitous in the

sense that no one was looking for it specifically. In 1811 Bernard Courtois (1777–1838), a French chemist, attempted to remove sodium and potassium compounds from the ash of burned seaweed in order to make gunpowder. After removing these chemicals from the ash,

he added sulfuric acid (H 2 SO 4 ) to the remaining ash. However, he mistakenly added too much acid, which produced a violet-colored vapor cloud that erupted from the mixture. This violet vapor condensed on all the metallic objects in the room, leaving a layer of solid black iodine crystals. Sir Humphry Davy (1778–1829) confirmed this discovery of a new element and named it iodine after the Greek word iodes, which means “violet,” but it was Courtois who was given credit for the discovery of iodine.