FLUORINE (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Fluorine is the lightest member of the halogen family, elements in Group 17 (VIIA) of the periodic table. The periodic table is a chart that shows how elements are related to one another. These include chlorine, bromine, iodine, and astatine. Fluorine is the most active chemical element, reacting with virtually every element. It even reacts with the noble gases at high temperatures and pressures. The noble gases, or Group 18 (VIIIA), also known as the inert gases, generally do not react with other elements.

Fluorine was discovered in 1886 by French chemist Henri Moissan (1852-1907). Moissan collected the gas by passing an electric current through one of its compounds, hydrogen fluoride (H₂F₂).

Consumers are most familiar with fluorine's use in two products. Fluorine gas is used to make fluorides, compounds that were made part of toothpastes in the 1950s. Fluorides are effective in preventing tooth decay and are added to urban water supplies as well.

SYMBOL
F

ATOMIC NUMBER
9

ATOMIC MASS
18.998404

FAMILY
Group 17 (VIIA)
Halogen

PRONUNCIATION
FLOR-een

Another group of fluorine compounds is the chlorofluorocarbons (CFCs). For many years, they were extremely popular as aerosol propellants. However, CFCs react with ozone (O₃) in the upper atmosphere. The ozone Layer filters harmful ultraviolet (UV) radiation from the Sun. Ultraviolet (UV) radiation is electromagnetic radiation (energy) of a wavelength just shorter than the violet (shortest wavelength) end of the visible light spectrum and thus with higher energy than visible light. As a result, the production of CFCs is now prohibited.

Discovery and naming

Chemistry has always been a dangerous science. Early chemistry was a hazardous occupation. Men and women worked with chemicals about which they knew little. The discovery of new compounds and elements could easily have tragic consequences.

Flourine was particularly vicious. Chemists suffered terrible injuries and even died before the element was isolated. Fluorine gas is extremely damaging to the soft tissues of the respiratory tract.

In the early 1500s, German scholar Georgius Agricola (1494-1555) described a mineral he called fluorspar. The name fluorspar comes from the Latin word fluere, meaning "to flow." Agricola claimed that fluorspar added to molten metallic ores made them more liquid and easier to work with. Although Agricola did not realize it, fluorspar was a mineral of fluorine and contains calcium fluoride (CaF₂).

Fluorspar became the subject of intense study by early chemists. In 1670, German glass cutter Heinrich Schwanhard discovered that a mixture of fluorspar and acid formed a substance that could be used to etch glass. Etching is a process by which a pattern is drawn into glass. The chemical reaction leaves a frosted image. Etching is used to produce artistic shapes on glass as well as precise scientific measuring instruments.

The new etching material was identified in 1771 by Swedish chemist Carl Wilhelm Scheele (1742-86). Scheele described, in detail, the properties of this material, hydrofluoric acid (HF). His work set off an intense study of the acid and its composition.

One goal was to find ways to break hydrofluoric acid into elements. Chemists suspected that one element had never been seen before. Little did they know, however, what a dangerous new element it would be. During studies of hydrofluoric acid, many chemists were disabled when they inhaled hydrogen fluoride gas. One chemist, Belgian Paulin Louyet (1818-50), died from his exposure to the chemical.

Finally, in 1888, the problem was solved. Moissan made a solution of hydrofluoric (HF) acid in potassium hydrogen fluoride (KHF₂). He then cooled the solution to -23°C (-9.4°F) and passed an electric current through it. A gas appeared at one end of the apparatus. He gave the name fluorine to the new element. The name comes from the mineral fluorspar.

Fluorine gas is extremely damaging to the soft tissues of the respiratory tract.

Physical properties

Fluorine is a pale yellow gas with a density of 1.695 grams per liter. That makes fluorine about 1.3 times as dense as air. Fluorine changes from a gas to a liquid at a temperature of -188.13°C (-306.5°F) and from a liquid to a solid at -219.61°C (-363.30°F).

Fluorine has a strong and characteristic odor that can be detected in very small amounts, as low as 20 parts per billion. This property is very helpful to those who work with fluorine. It means that the gas can be detected and avoided in case it leaks into a room.

Chemical properties

Fluorine is the most reactive element. It combines easily with every other element except helium, neon, and argon. It reacts with most compounds, often violently. For example, when mixed with water, it reacts explosively. For these reasons, it must be handled with extreme care in the laboratory.

Occurrence in nature

Fluorine never occurs as a free element in nature. The most common fluorine minerals are fluorspar, fluorapatite, and cryolite. Apatite is a complex mineral containing primarily calcium, phosphorus, and oxygen, usually with fluorine. Cryolite is also known as Greenland spar. (The island of Greenland is the only commercial source of this mineral.) It consists primarily of sodium aluminum fluoride (Na₃ALF₆). The major sources of these minerals are China, Mexico, Mongolia, and South Africa. The United States once produced small amounts of fluorspar, but its last remaining mine closed in 1995. The United States now imports the fluorine minerals it needs.

Fluorine is an abundant element in the Earth's crust, estimated at about 0.06 percent in the earth. That makes it about the 13th most common element in the crust. It is about as abundant as manganese or barium.

Isotopes

There is only one naturally occurring isotope of fluorine, fluroine-19. 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.

Only one radioactive isotopes of fluorine, fluorine-18, has been prepared. 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.

Fluroine-18 is sometimes used for medical studies. It is injected into the body where it travels primarily to bones. Its presence in bones can be detected by the radiation it gives off. The radiation pattern discloses how normal bones are. Fluorine-18 is sometimes used in a similar way to study brain function.

Extraction

Fluorine is commercially made by Moissan's method. An electric current is passed through a mixture of hydrogen fluoride and potassium hydrogen fluoride:

Uses and compounds

Fluorine has relatively few uses as an element. It is much too active for such applications. One use of elemental fluorine is in rocket fuels. It helps other materials burn, like oxygen does. The greatest majority of fluorine is used to make compounds of fluorine.

Fluorides are compounds of fluorine with a metal. Sodium fluoride (NaF), calcium fluoride (CaF₂), and stannous fluoride (SnF₂) are examples of fluorides.

A familiar use of some fluoride compounds is in toothpastes. Studies show that small amounts of fluorides can help reduce tooth decay. Fluorides are deposited as new tooth material is formed, making it strong and resistant to decay.

Some cities add fluorides to their water supply. By doing so, they hope to improve the dental health of everyone living in the city. Young people, whose teeth are still developing, benefit the most. The process of adding fluorides to public water supplies is called fluoridation. Too much fluorine in the water leads to a light brown and permanent staining.

Because of fluorine's strong and characteristic odor, it can be detected and avoided in case it leaks into a room.

Some people worry about the long-term health effect of fluorides added to public water supplies. They point out that fluorine is a deadly poison and that fluorides can be toxic as well. It is true that fluorine gas is very toxic, but the properties of compounds are different than the elements involved. There is little evidence to support these concerns.

Fluorides tend to be dangerous only in large doses. The amount of fluoride added to public water supplies is usually very small, only a few parts per million. Most dental and health experts believe that fluoridation is a helpful public health practice, not a threat to the health of individuals.

Slip, sliding around

S erendipity plays a big part in scientific research. The term serendipity means a discovery made by accident. One of the most profitable discoveries made this way is of the material known as Teflon. Teflon is the trade name of a kind of plastic made by the DuPont Chemical Company. It has become an important commercial product for one main reason: very few things stick to Teflon. Most kitchen cupboards probably contain skillets with cooking surfaces covered with Teflon. Most food will not stick to the pans as they cook. And Teflon pans need no oil or butter.

Teflon was discovered by accident in 1938 by a DuPont chemist named Roy Plunkett (1911-94). Plunkett was working on the development of chlorofluorocarbons (CFCs) for DuPont. He wanted to see what happened when one compound, tetrafluoroethylene, or TFE (C₂F₄), was mixed with hydrochloric acid (HCl). To carry out the experiment, he set up the equipment so that the gaseous TFE would flow into a container of HCl.

When he opened the valve on the TFE container, however, nothing came out! Plunkett could have discarded the tank, but he didn't. Instead, he sawed open the container. Inside he found that the TFE had polymerized into a single mass. Polymerization is the process by which many thousands of individual TFE molecules join together to make one very large molecule. The large molecule is called polytetrafluorethylene, or PTFE.

Plunkett scraped the white PTFE powder out and sent it to DuPont scientists working on artificial fibers. The scientists studied the properties of PTFE. They discovered its non-stick qualities and were soon working on a number of applications for the new material.

DuPont registered the Teflon trademark in 1945 and released its first Teflon products a year later. Since then the non-stick coating has become a household staple on kitchen cookware, in baking sprays, and as a stain repellant for fabrics and textiles.

Studies show that small amounts of fluorides in toothpaste can help reduce tooth decay. Fluorides are deposited as new tooth material is formed, making it strong and resistant to decay.

The second major use of fluorine was in the production of CFCs. CFCs were discovered in the late 1920s by American chemical engineer Thomas Midgley, Jr. (1889-1944). These compounds have a number of interesting properties. They are very stable and do not break down when used in a variety of industrial operations. They were widely used in cooling and refrigeration systems, as cleaning agents, in aerosol sprays, and in specialized polymers. The production of CFCs grew from about 1 million kilograms in 1935 to more than 300 million kilograms in 1965 to more than 700 million kilograms in 1985.

By the mid-1980s, however, evidence showed that the compounds were damaging the Earth's ozone layer. This layer lies at an altitude of 20 to 50 kilometers (12 to 30 miles) above the Earth's surface. It is important to life because it shields the Earth from the Sun's harmful ultraviolet radiation. These concerns led to CFCs being phased out. The compounds are no longer produced or used in the United States and most other parts of the world. Substitutes that are safer to the Earth are now being used in the products that once used CFCs.

Breaking up is hard to do

P eople are often confused about CFCs. They were popular industrial chemicals because they do not break down very easily. They were long used in most car air conditioners as a heat transfer fluid. CFCs took heat out of the car and moved it into the outside air. This process was carried out over and over again. CFCs in a car's system lasted a very long time.

But scientists realized that CFCs are a threat to the ozone layer because they break down. How can that be?

There is always a little leakage from auto air conditioning units and from every device in which CFCs are used. CFCs are gases or liquids that easily vaporize and float upward into the atmosphere. They eventually reach the ozone layer.

At this height above the Earth's surface, CFCs encounter intense radiation from the Sun and break apart. A molecule that was stable near the Earth's surface has now become unstable.

When a CFC molecule breaks apart, it forms a single chlorine atom:

(The CFC* means a CFC molecule without one chlorine atom.) The chlorine formed in this reaction can react with an ozone (O₃) molecule:

Here's the problem: The ozone (O₃) will filter out harmful radiation from the Sun. It removes most of the radiation that causes severe sunburns and skin cancer. But oxygen (O₂) cannot do the same thing.

Therefore, (1) the more CFCs in the atmosphere, the more chlorine atoms; (2) the more chlorine atoms, the fewer ozone molecules; (3) the fewer ozone molecules, the more harmful radiation reaching the Earth's surface; and (4) the more harmful radiation, the more cases of skin cancer and other health problems.

It is this series of events that convinced politicians to ban the further production and use of CFCs.

By the mid-1980s, evidence showed that CFCs were damaging the Earth's ozone layer.

Health effects

As noted under "Discovery and naming," fluorine can be quite dangerous. If inhaled in small amounts, it causes severe irritation to the respiratory system (nose, throat, and lungs). In larger amounts, it can cause death. The highest recommended exposure to fluorine is one part per million of air over an eight-hour period.

fluor·ine / ˈfloŏrˌēn; flôr-/ • n. the chemical element of atomic number 9, a poisonous pale yellow gas of the halogen series. It is the most reactive of all the elements, causing severe burns on contact with skin. (Symbol: F)