If you have ever been out at night in a large town or city, you probably have seen colored lights on stores and advertising signs. These are called neon lights, named for the gas, neon, that many of them contain. But not all of them are lighted by glowing neon gas. Other gases—such as helium, argon, krypton, and xenon—are also used in the lights.
Each of these gases gives out a different-colored light when electricity is sent through it. The color of light given out by the gas will vary, depending on such things as temperature, pressure, and electric voltage. Neon gives out red-orange light; argon gives out a reddish blue light. The light from helium is white, yellow, or sometimes violet; from krypton it is yellow, green, or pale violet; and from xenon, either blue or blue-green.
These gases form a family of elements known as the noble (sometimes called inert) gases. A sixth gas, radon, is also included in this group. Radon is a radioactive gas formed by the nuclear reactions of radium, thorium, and other radioactive elements.
The noble gases were once thought to be chemically inert—that is, scientists believed that these elements were totally incapable of combining with other elements to form compounds. We now know that these gases are not completely inactive but instead have a very low degree of reactivity. Chemists are able to prepare compounds of krypton and xenon with fluorine and compounds of xenon with oxygen under special laboratory conditions.
Sometimes the noble gases are called the rare gases, though some of them are far more abundant than many other elements. Whichever name is used, their most important characteristic is that they are all relatively inactive chemically. They form no chemical compounds under normal conditions.USES OF NOBLE GASES
Besides providing colored light for signs, noble gases have a variety of other uses. Neon is used in the sodium-vapor lamps that light highways, in automobile spark plug testers, and in many electric tube devices.
Helium, the lightest noble gas, is used in balloons, blimps, and dirigibles. It has replaced hydrogen for this purpose because it is safe—it will not burn or explode. A nonflammable mixture of hydrogen and helium is also used. Helium is one of the noble gases used to protect metals that cannot be exposed to air when they are being welded. Welded joints may not be strong unless clean metal, free from oxygen compounds, is joined. The electronics industry uses helium in this way in manufacturing transistors.
Argon is also used as a protective gaseous shielding around metals that are being welded. One of the chief uses of argon, however, is in electric light bulbs. The argon slows down the evaporation of the tungsten filament in the bulb when it gets hot.
Krypton and xenon, although very rare gases, are widely used in photographic equipment, especially in flashbulbs. Both gases give a brilliant flash of light when electricity is sent through them. Such lamps may last for 10,000 flashes. Like argon, krypton slows down the evaporation of the filament in electric light bulbs. The bright lamps that light airport runways may be filled with krypton. Krypton-85 is a radioactive type of krypton gas that serves as a tracer gas in checking pipelines and other equipment for leaks. The radioactive gas can be detected by a Geiger counter and followed through the pipeline.
Xenon is often used in electron tubes. It can be made into a good conductor of electricity if its pressure is reduced and a high enough electric voltage is used. The noble gases do not conduct electricity at ordinary pressures.
Because of its radioactivity, radon is especially useful in the treatment of cancer and in biological research.HOW NOBLE GASES ARE OBTAINED
The chief source of the noble gases is ordinary air (except for helium, which is obtained from natural gas). The gases are mixed in the air together with oxygen, nitrogen, carbon dioxide, and other substances.
To obtain noble gases, ordinary air is first liquefied. Each gas is then removed from the air one at a time. This is done by chilling the air to a very low temperature, about −330°F (−200°C), which liquefies it. The liquid air is then piped into tall towers and slowly heated. As each gas reaches its boiling point, it boils off from the liquid air as a gas. Each gas has a different boiling point and can thus be drawn off separately. This method of separation is known as fractional distillation.The Problem of Radon. Radon occurs naturally in the ground, especially near uranium deposits. (Uranium slowly breaks down to form radium.) In some regions, radon seeps into homes, where—if ventilation is poor—it accumulates and poses a health hazard to the occupants. In high concentrations, radon greatly increases a person's chances of developing lung cancer, but researchers are not sure exactly what levels of exposure are dangerous. Changes in construction practices plus repairs to existing homes may help prevent harmful accumulations of this radioactive gas. Special tests can determine if harmful levels have been reached.
DISCOVERY OF THE NOBLE GASES
Unlike some elements that have been known for hundreds of years, the noble gases were not discovered until the end of the 1800's. They escaped the notice of scientists for a long time for several reasons. For one, the gases make up only a tiny fraction of the Earth's atmosphere. Argon, the most plentiful noble gas, is 0.9 percent of the atmosphere, but it was difficult to find because it is inactive chemically. In fact, the name "argon" was chosen because of its chemical inertness. Its name comes from the Greek word for "lazy." The other noble gases are found in much smaller quantities in the air. Xenon makes up less than 1 part in 10 million parts of air.
Another reason that the noble gases were unknown for a long period is that they have no color, except when they give off colored light as an electric current goes through them. In addition, they have no odor or taste, and they do not burn. Unlike most other elements, they are not found combined with other substances in nature.
The most important work in discovering and isolating the noble gases (except radon) was done by the British chemists Sir William Ramsay and Lord Rayleigh. Argon was discovered by them in 1894. Helium was isolated from uranium ore by Ramsay in 1895. However, evidence of its existence in space was found earlier by astronomers studying light from the sun during the solar eclipse of 1868. Helium is the only element that was discovered in space before it was discovered on Earth.
Neon, krypton, and xenon were isolated in 1898 by Ramsay and Morris William Travers. Radon was isolated in 1900. Since that time the noble gases have become important in many areas of industry.
Elbert C. Weaver
Chemistry for Our Times