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Iodine                                                                                                                      

Standard Sate: solid at 298K 

Color: violet-dark grey, lustrous                    

Iodine is a bluish-black, lustrous solid. It volatilises at ambient temperatures into a pretty blue-violet gas with an irritating odour. 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.

(I), chemical element, a member of the halogen elements, or Group
VIIa of the periodic table.

Iodine is a nonmetallic, nearly black crystalline solid that is used in
medicine, in the synthesis of some organic chemicals, in the
manufacture of dyes, in analytical chemistry, and in photography. At
room temperature, solid iodine sublimes to a deep violet vapour that
is irritating to the eyes, nose, and throat. Iodine dissolves in alcohol
and slightly in water to give brown solutions and in carbon
tetrachloride and carbon disulfide to give violet solutions.

Iodine is never found in nature uncombined. It is sparingly present in
seawater as iodide ion, I-, to the extent of approximately 50 milligrams
of iodine per ton of seawater. Appreciable quantities are found in a
number of natural brines. Iodine occurs in animals and plants,
generally in very small amounts, but very abundantly in seaweeds.
Dispersed as a trace element in rocks, iodine is not sufficiently
concentrated to form independent minerals. A formerly important
source of iodine for commercial preparation is the saltpetre or nitrate
deposits of Chile, in which iodine is present as solid iodates,
especially calcium iodate, Ca(IO3)2. A French manufacturer of
saltpetre from seaweeds, Bernard Courtois, discovered (1811) iodine
in water that was used to extract the soluble material from seaweed
ash. Since that time, in France and Great Britain and, more recently,
in Japan, iodine has been recovered from seaweeds.

Iodine is an essential microconstituent in the human body, which
contains an average of 14 mg (0.00049 ounce) of the element,
concentrated mostly in the thyroid gland. The thyroid gland secretes
iodine-bearing hormones, especially thyroxine, which are essential
for maintaining normal metabolism in all the body's cells. Where
insufficient iodine is present in the food supplies, as in North
America's Great Lakes region, The Netherlands, and most
mountainous areas of the world, the main source of iodine is table
salt to which potassium iodide or other iodizing chemicals have been
added. Iodine deficiency leads to endemic goitre and myxedema.

Iodine at high concentration is poisonous and may cause serious
damage to skin and tissues. In dilute alcoholic solution (tincture of
iodine) or aqueous solution, it has limited use as a topical antiseptic.

In chemical compounds the common oxidation states are -1 (iodides),
+5 (iodates), and +7 (periodates). Iodine combines readily with most
metals and some nonmetals to form iodides. The iodide ion is a strong
reducing agent; that is, it readily gives up one electron. Although the
iodide ion is colourless, iodide solutions may acquire a brownish tint as
a result of oxidation of iodide to free iodine by atmospheric oxygen.
Molecules of elemental iodine, consisting of two atoms (I2), combine
with iodides to form polyiodides (typically I2 + I- I-3), accounting
for the high solubility of iodine in solutions containing soluble iodide.
The aqueous solution of hydrogen iodide (HI), known as hydriodic
acid, is a strong acid that is used to prepare iodides by reaction with
metals or their oxides, hydroxides, and carbonates. Iodine exhibits a
+5 oxidation state in the moderately strong iodic acid (HIO3), which
can be readily dehydrated to yield the white solid iodine pentoxide
(I2O5). Periodates may take a form represented by potassium
metaperiodate (KIO4) or silver paraperiodate (Ag5IO6), for example,
because the large size of the central iodine atom allows a relatively
large number of oxygen atoms to get close enough to form bonds.

The only naturally occurring isotope of iodine is stable iodine-127.
An exceptionally useful radioactive isotope is iodine-131, which has a
half-life of eight days. It is employed in medicine to monitor thyroid
gland functioning, to treat goitre and thyroid cancer, and to locate
tumours of the brain and of the liver. It is also used in investigations to
trace the course of compounds in metabolism. Several iodine
compounds are used as contrast mediums in diagnostic radiology. In
aqueous solution even minute amounts of iodine in the presence of
starch produce a blue-black colour. atomic number 53 atomic weight
126.9044 melting point 113.5 C (236 F) boiling point 184 C (363 F)
specific gravity 4.93 (20 C) oxidation states -1, +1, +3, +5, +7
electronic config. 2-8-18-18-7 or (Kr)4d105s25p5

"iodine" Encyclop�dia Britannica Online.

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