Iodine 3326
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Iodine is the heaviest of the commonly occurring halogens. The halogens are in Group 17 (VIIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. Iodine's chemical properties are similar to the lighter halogens above it, fluorine, chlorine, and bromine. But its physical appearance is very different. It is a steel-gray solid that changes into beautiful purple vapors when heated.

Iodine was discovered in 1811 by French chemist Bernard Courtois (1777-1838). The element occurs primarily in seawater and in solids formed when seawater evaporates. Its single most important property may be the ability to kill germs. It is used in antiseptics, germicides (products that kill germs), and other medical applications. However, it has a great many other less common, but important, commercial applications.




Group 17 (VIIA)


Discovery and naming

One of Courtois' first jobs was to assist his father in making compounds of sodium and potassium from seaweed. Seaweed plants take sodium and potassium compounds out of seawater. The compounds become part of the growing seaweed.

Courtois and his father collected seaweed on the coasts of Normandy and Brittany in France. Then they burned it. Next, they soaked the seaweed ashes in water to dissolve the sodium and potassium compounds. Sulfuric acid was added to react with the unwanted seaweed chemicals. Finally, they allowed the water to evaporate, leaving the compounds behind. These compounds are white crystals, much like ordinary table salt. The compounds were sold to large industrial businesses for use in such products as table salt and baking soda.

One day in 1811, Courtois made a mistake. He added too much sulfuric acid to the mixture. He was amazed to see clouds of beautiful violet vapor rising from the mixture. He decided to study the new material. Eventually, he proved it was a new element. He named the element after its color. In Greek, the word iodes means "violet."

Physical properties

Iodine is one of the most striking and beautiful of all elements. As a solid, it is a heavy, grayish-black, metallic-looking material. When heated, it does not melt. Instead, it sublimes. Sublimation is the process by which a solid turns directly to a gas without first melting. The resulting iodine vapor has a violet color and a harsh odor. If a cold object, such as an iron bar, is placed in these vapors, iodine changes back to a solid. It forms attractive, delicate, metallic crystals.

Iodine dissolves only slightly in water. But it dissolves in many other liquids to give distinctive purple solutions. If heated under the proper conditions, it can be made to melt at 113.5°C (236.3°F) and to boil at 184°C (363°F). The density of the element is 4.98 grams per cubic centimeter.

Chemical properties

Like the other halogens, iodine is an active element. However, it is less active than the three halogens above it in the periodic table. Its most common compounds are those of the alkali metals, sodium, and potassium. But it also forms compounds with other elements. It even forms compounds with the other halogens. Some examples are iodine monobromide (IBr), iodine monochloride (ICl), and iodine pentafluoride (IF 5 ).

A magnified view of a crystal of iodine.
A magnified view of a crystal of iodine.

Occurrence in nature

Iodine is not very abundant in the Earth's crust. Its abundance is estimated to be about 0.3 to 0.5 parts per million. It ranks in the bottom third of the elements in terms of abundance. It is still more common than cadmium, silver, mercury, and gold. Its abundance in seawater is estimated to be even less, about 0.0003 parts per million.

Iodine tends to be concentrated in the Earth's crust in only a few places. These places were once covered by oceans. Over millions of years, the oceans evaporated. They left behind the chemical compounds that had been dissolved in them. The dry chemicals left behind were later buried by earth movements. Today, they exist underground as salt mines.

A mistake by Bernard Courtois led to clouds of beautiful violet vapor rising from a mixture on which he was working. It was iodine.

Iodine can also be collected from seawater, brackish water, brine, or sea kelp. Seawater is given different names depending on the amount of solids dissolved in it. Brackish water, for

Kelp, a type of seaweed, is a popular source of iodine, since it absorbs the element from seawater.
Kelp, a type of seaweed, is a popular source of iodine, since it absorbs the element from seawater.
example, has a relatively low percentage of solids dissolved in water. The range that is sometimes given is 0.05 to 3 percent solids in the water. Brine has a higher percentage of dissolved solids. It may contain anywhere from 3 to 20 percent of solids dissolved in water.

Finally, sea kelp is a form of seaweed. As it grows, it takes iodine out of seawater. Over time, sea kelp has a much higher concentration of iodine than seawater. Sea kelp is harvested, dried, and burned to collect iodine. The process is not much different from the one used by Courtois in 1811.

Iodine compounds are used in the production of photographic film.
Iodine compounds are used in the production of photographic film.


Only one naturally occurring isotope of iodine is known, iodine-127. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Approximately 30 radioactive isotopes of iodine have been made artificially. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

A number of iodine isotopes are used commercially. In medical applications, these isotopes are injected into the body or given to the patient through the mouth. The isotopes then travel through the body in the bloodstream. As they travel, they give off radiation. That radiation can be detected by using X-ray film. A medical specialist can tell how well the body is functioning by observing the pattern of radiation.

Iodine isotopes are used in many ways. Iodine-123 is used in studies of the brain, kidneys, and thyroid. Iodine-125 is used in studies of the pancreas, blood flow, thyroid, liver, take-up of minerals in bones, and loss of proteins in the body. And iodine-131 is used in studies of the liver, kidneys, blood flow, lungs, brain, pancreas, and thyroid.

Iodine and human health

T he amount of iodine in the human body is very small. To find out how much is in one's body, one's body weight is divided by 2,500,000. That number is the weight of iodine in the body. For normal people, the amount is about equal to the size of the head of a pin.

That tiny dot of iodine can mean the difference between good and bad health. People who do not have enough can develop serious health problems. At one time, the most common of those problems was a disease known as goiter. Goiter causes a large lump in the neck as the thyroid grows out of control. (It can grow as large as a grape-fruit.) A goiter tries to make thyroid hormones, but it does not receive enough iodine from the person's diet. So it keeps expanding, trying to do its job.

A lack of iodine can cause other problems too. For example, thyroid hormones are needed for normal brain development in an unborn child. They are also needed to continue that development after birth. People who do not include enough iodine in their diet do not develop normally. Today, experts say that low levels of iodine are the leading cause of mental retardation, deafness, mutism (the inability to speak), and paralysis. They also say less serious problems can be blamed on low iodine levels. These include lethargy drowsiness, clumsiness, and learning disabilities.

Low iodine levels can be easily corrected. In most developed countries today, companies that make table salt add a small amount of potassium iodide (KI) to their salt. The salt is labeled "iodized salt." People who use it get all the iodine they need for normal thyroid function.

But people who live in developing countries may not be able to get iodized salt. The World Health Organization (WHO) is trying to find ways of providing iodine to these people. The WHO estimates that 1.5 billion people live in areas where levels of iodine are low. Up to 20 million of these people may have mental disabilities because of a lack of iodine. The WHO has started a program to ensure that future generations in these regions get the iodine needed to develop and function normally.

The most common iodine isotope used is iodine-131. When iodine (of any kind) enters the body, it tends to go directly to the thyroid. The iodine is then used to make thyroid hormones. If radioactive iodine is used, a doctor can tell how well the thyroid gland is working. If a high level of radiation is given off, the gland may be overactive. If a low level of radiation is given off, the gland may be underactive. In either case, the person may need some treatment to help the thyroid gland work more normally.


When a mixture of substances containing iodine is heated, the iodine sublimes. It can then be collected and purified.

Uses and compounds

About two-thirds of all iodine and its compounds are used in sanitation systems or in making various antiseptics and drugs. Iodine is also used to make dyes, photographic film, and specialized soaps. It is used in some industries as a catalyst. A catalyst is a substance used to speed up or slow down a chemical reaction. The catalyst does not undergo any change itself during the reaction.

Health effects

Iodine can have both favorable and unfavorable effects on living organisms. It tends to kill bacteria and other disease-causing organisms. In fact, this property leads to its use in sanitation systems and as an antiseptic. An antiseptic is a chemical that stops the growth of germs. Not so long ago, tincture of iodine was one of the most popular antiseptics. It was applied to cuts and wounds to prevent infection. Tincture is a solution made by dissolving some substance (such as iodine) in alcohol rather than in water. Today, tincture of iodine has been replaced by other antiseptics.

One reason that tincture of iodine is used less today is that it can also cause problems. In higher doses, iodine can irritate or burn the skin. It can also be quite poisonous if taken internally.

Iodine kills bacteria and other disease-causing organisms.

Iodine plays an important role in the health of plants and animals. It is needed to maintain good health and normal growth. In humans, iodine is used to make a group of important compounds known as thyroid hormones. These chemicals are produced in the thyroid gland at the base of the neck. These chemicals control many important bodily functions. A lack of thyroid hormones can result in the disorder known as goiter. Goiter causes a large lump in the neck as the thyroid grows out of control. Iodine is added to table salt today, so goiter is rarely seen in the United States.

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