Halogens

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Photo by: Jedi Master

The halogens are the five chemical elements that make up Group 17 on the periodic table: fluorine, chlorine, bromine, iodine, and astatine. The term halogen comes from Greek terms meaning "to produce sea salt." The halogens are all chemically active. For that reason, none occur naturally in the form of elements. However—with the exception of astatine—they are very widespread and abundant in chemical compounds. The most widely known of these compounds is sodium chloride, or common table salt.

Fluorine and chlorine are gases. Bromine is one of only two liquid elements. Iodine is a solid. Astatine is radioactive and is one of the rarest of the chemical elements. Fluorine is the most reactive of all known elements. Chemical reactivity decreases throughout this family of elements, with fluorine being the most reactive of all known elements, and chlorine, bromine, and iodine being relatively less reactive, respectively.

Simple compounds of the halogens are called halides. When a halogen becomes part of a compound with one other element, its name is changed to an -ide ending; for example, a chloride.

Fluorine

The name fluorine comes from the name of the mineral in which the element was found, fluorspar. Fluorine was one of the last common elements to be isolated. It is so reactive that chemists searched for more than 70 years to find a way to extract the element from its compounds. Then, in 1886, French chemist Henri Moissan (1852–1907) found a way to produce fluorine by passing an electric current through a liquid mixture of potassium fluoride and hydrogen fluoride. Moissan's method is still used today, with some modifications, for the production of fluorine.

Properties and uses. Fluorine is one of the most dangerous chemicals known. It attacks the skin and throat, causing serious burns and respiratory problems at very low concentrations. It is also very reactive chemically. It attacks most chemicals vigorously at room temperature and reacts explosively with water.

An indication of fluorine's reactivity is that it even forms compounds with the family of elements known as the inert gases. The inert gases include helium, neon, argon, krypton, and xenon. They get their name from the fact that they generally do not combine with any other element. However, compounds of xenon and fluorine and krypton and fluorine have been produced. They are the only known compounds of the inert gases to have been discovered.

Because it is so reactive, fluorine itself has few uses. One exception is its role as an oxidizing (burning) agent in rocket fuels. The vast majority of fluorine, however, is used to make compounds. One of the most interesting of those compounds is hydrofluoric acid. This compound has been used since the 1600s to etch glass.

Perhaps the most familiar application of fluorine compounds is in toothpaste additives. Scientists have discovered that the addition of tiny amounts of fluoride in a person's diet can decrease the number of dental caries (cavities) that develop. Today, many kinds of toothpastes include stannous fluoride to improve a person's dental health.

For many years, the most important group of fluorine compounds used commercially were the chlorofluorocarbons (CFCs). The CFCs were developed and used as refrigerants, blowing agents for polyurethane foam, and propellants in spray cans. At one time, more than 700 million kilograms (1.5 billion pounds) of CFCs were produced in a single year.

In the 1980s, however, scientists found that CFCs break down in the atmosphere. The chlorine formed as a result of this breakdown attacks the ozone layer in Earth's stratosphere (the part of Earth's atmosphere that extends 7 to 31 miles [11 to 50 kilometers] above the surface). The loss of the ozone layer is a serious problem for humans since ozone screens out radiation that causes skin cancer and other damage to plants and animals on Earth. Today, scientists are exploring the use of another class of fluorine compounds—the hydrochlorofluorocarbons, or HCFCs—as replacements for CFCs.

Chlorine

Chlorine was first prepared in the 1770s by Swedish chemist Carl Wilhelm Scheele (1742–1786), who thought it was a compound. It was later identified as an element by English chemist Humphry Davy (1778–1829). Davy suggested the name of chlorine for the element because of its greenish-yellow color. (The Greek word for "greenish-yellow" is chloros. )

Chlorine occurs most abundantly in sodium chloride, which is obtained from seawater and from underground deposits of rock salt formed from seas that have dried up. To obtain chlorine, an electrical current is passed through brine, a water solution of sodium chloride.

Properties and uses. Chlorine gas is toxic. It attacks the respiratory tract, causing coughing, congestion, and flu-like symptoms. In high doses, it can be fatal. For this reason, chlorine was used as a chemical weapon during World War I (1914–18).

Chlorine is also very reactive, although less so than fluorine. It forms compounds with almost every other element. Among the most important of those compounds are sodium chloride (table salt), potassium chloride, hydrochloric acid, and calcium chloride.

Chlorine consistently ranks among the top ten chemicals produced in the United States. Some of chlorine's uses depend on its toxic effects. For example, chlorine is now widely used as a disinfectant in municipal water systems, swimming pools, and sewage treatment plants. Many organic (carbon-containing) compounds of chlorine are used as pesticides, herbicides, and fungicides. These compounds kill unwanted insects, weeds, fungi, and other plants and animals. The use of these compounds is often associated with undesirable environmental effects, however. DDT, for example, is a chlorine-containing compound that was once one of the most popular pesticides ever produced. But its harmful effects on fish, birds, and other animals in the environment eventually led to bans on its use in many industrialized nations.

Chlorine is also used in the bleaching of paper, pulp, and textiles. The largest single application of the element is in the preparation of a large variety of compounds, including organic chlorides that are the starting point in the manufacture of plastics and other kinds of polymers (chemical compounds that consist of repeating structural units). One of the most important of these polymers is polyvinyl chloride (PVC), from which plastic pipe and many other plastic products are made. Another is neoprene, a synthetic form of rubber that is resistant to the effects of heat, oxidation, and oils. Neoprene is widely used in automobile parts.

Bromine

Bromine was discovered in 1826 by French chemist Antoine-Jérôme Balard (1802–1876). Balard chose the name bromine from the Greek word for "stink," because of its strong and disagreeable odor. Like chlorine, bromine is obtained from brine. Chlorine gas is used to convert bromide compounds in brine to elemental bromine.

Properties and uses. Bromine is a beautiful reddish-brown liquid that vaporizes (changes to a gas) easily. The vapors are irritating to the eyes and throat. The liquid is highly corrosive and can cause serious burns if spilled on the skin. Bromine is chemically less active than fluorine and chlorine but more active than iodine.

Like chlorine, bromine can be used as a disinfectant. In fact, some water treatment systems have converted from chlorination to bromination as a way of purifying water. For many years, one of the most important compounds of bromine was ethylene dibromide, an additive in leaded gasolines. Since leaded gasoline has been removed from the market, this use has declined.

The product in which most people are likely to encounter compounds of bromine is in photographic film. Tiny crystals of silver bromide undergo a chemical change when exposed to light. This change is responsible for the image produced when photographic film is used to take a picture.

Bromine is also used to make a number of organic products that function as pesticides. The most popular of these currently is methyl bromide, a fumigant (another word for a substance used to destroy pests). Methyl bromide is used as a spray for potatoes, tomatoes, and other agricultural crops.

The halons are a group of organic compounds that contain bromine along with at least one other halogen. The halons are popular as flame retardants. However, scientists have found that, like the CFCs, they appear to cause damage to Earth's ozone layer. For that reason, their use has been largely reduced throughout the world.

Iodine

Iodine was discovered accidentally in 1811 by French chemist Bernard Courtois (1777–1838). Courtois was burning seaweed to collect potassium nitrate when he noticed that a beautiful violet vapor was produced. When the vapor cooled, it changed to dark, shiny, metallic-like crystals. Humphry Davy later suggested the name iodine for the element from the Greek word iodos, for "violet."

As with chlorine and bromine, iodine is obtained from seawater. It can also be produced from Chile saltpeter (sodium nitrate), in which it occurs as an impurity in the form of sodium iodate (NaIO ₃ ).

Properties and uses. Iodine vapor is irritating to the eyes and respiratory system. It is highly toxic if ingested. Iodine is the least active of the common halogens (not counting astatine).

The human body uses iodine to make thyroxine, an important hormone (chemical messenger) produced by the thyroid gland. (The thyroid is a gland located in the neck that plays an important role in metabolism—a term used to describe processes of energy production and use by the body.) If insufficient amounts of iodine are present in the diet, a person may develop a condition known as goiter, a sometimes noticeable enlargement of the thyroid gland. Once the relationship between iodine and goiter were first discovered, manufacturers of table salt began to add iodine (in the form of sodium iodide) to their product (iodized salt). This practice has largely eliminated the problem of goiter in modern developed nations.

Iodine is also used commercially in a variety of products including dyes, specialized soaps, lubricants, photographic film, medicines, and pharmaceuticals.

Astatine

Astatine is generally regarded as one of the rarest naturally occurring elements. According to some estimates, no more than 44 milligrams of the element are to be found in Earth's crust. It is hardly surprising, then, that the element was first produced synthetically. In 1940, three physicists at the University of California at Berkeley—D. R. Corson, K. R. Mackenzie, and Emilio Segrè (1905–1989)—made astatine by bombarding the element bismuth with alpha particles in a cyclotron (a particle accelerator or atom-smasher).

About 24 isotopes (forms) of astatine exist, all of them radioactive. The most long-lived has a half-life of 8.3 hours, meaning that half of a sample of the element disappears in 8.3 hours. Because it is so rare and has such a short half-life, astatine is one of the most poorly understood of all chemical elements. It has no practical applications at this time.

[ See also Element, chemical ; Organic chemistry ; Periodic table ]