Inert and noble gases
Inert and noble gases are a group of chemically inert, colorless, tasteless, and odorless monoatomic gases that are found in the Earth's atmosphere and various natural sources. This group includes helium, neon, argon, krypton, xenon, and radon. These gases occur naturally in the atmosphere, with argon being the most abundant, representing approximately 0.937% of dry air, while neon, krypton, and xenon are found in much smaller amounts. Noble gases are primarily used in applications such as arc welding, lighting (neon and fluorescent lights), and lasers, and they serve as inert atmospheres in various industrial processes.
Despite their name suggesting inactivity, they can form compounds under extreme conditions, particularly xenon and krypton, while radon is notably radioactive and poses health risks due to its accumulation in homes. The discovery and isolation of these gases date back to the late 19th century, with significant contributions from scientists like Lord Rayleigh and Sir William Ramsay. Their properties and uses have implications in fields ranging from geology to medical applications, making them significant both scientifically and industrially.
Inert and noble gases
Where Found
The noble gases—neon, argon, krypton, helium, radon, and xenon—naturally compose a small part of the atmosphere. The gases are also found in hot-spring water. Argon has been found in certain rocks with helium. Helium is addressed in its own entry and therefore is not covered here. Radon gas, which is ubiquitous, is an end product of uranium decay that is radioactive and emanates from soil, rocks, and hot springs in areas where uranium and thorium are found.
Primary Uses
The primary uses of these gases are in arc welding, neon lights, fluorescent lights, and lasers. They are also used as Geiger counters and inert atmospheres.
Technical Definition
The inert or noble gases are Group VIIIA of the periodic table of the elements. They are colorless, tasteless, and odorless monoatomic gases.
Description, Distribution, and Forms
Neon (abbreviated Ne), atomic number 10, has three naturally occurring stable isotopes: neon 20 (90.51 percent), neon 21 (0.27 percent), and neon 22 (9.22 percent). The atomic weight is 20.183, with a boiling point of -246° Celsius and a melting point of -249° Celsius. Argon (Ar), atomic number 18, has three naturally occurring stable isotopes: argon 40 (99.600 percent), argon 38 (0.0632 percent), and argon 36 (0.3364 percent). The atomic weight is 39.944, with a boiling point of -186° Celsius and a melting point of -189° Celsius.
Krypton (Kr), atomic number 36, has six naturally occurring stable isotopes (78, 80, 82, 83, 84, and 86), of which 84 is the most abundant (57.0 percent). The atomic weight is 83.80, with a boiling point of -157° Celsius and a melting point of -153° Celsius. One that has been studied, krypton 85, is mainly generated in uranium reprocessing plants but also in nuclear reactors and as a product of spontaneous fission. The study concluded that the concentration could grow to the point that krypton 85 could produce as much radiation exposure for humans as is the natural background radiation. The outcome of this could be an increase in skin cancer.
Xenon (Xe), atomic number 54, has nine naturally occurring stable isotopes. The atomic weight is 131.30, with a boiling point at -112° Celsius and a melting point of -107° Celsius.
Although argon has been found in certain igneous rocks with helium, and all the gases have been found in water from hot springs, the is still the major source of the noble gases. Dry air contains 0.937 percent (9,370 parts per million) argon, 18 parts per million neon, 1.1 part per million krypton, and 0.086 part per million xenon. The higher concentration of argon is thought to be because radioactive potassium 40 decays to argon. The group has been called rare gases or inert gases, but since the atmosphere is almost 1 percent argon, and because krypton and xenon are not totally inert, the name “noble gases” has gained favor. The noble gases are always found as inert, monoatomic gases. Although compounds of xenon and krypton have been formed, they can be formed only under extreme conditions; no compounds occur naturally.
Radon (Rn), atomic number 86, is a decay product of radium and occurs in nature as a very dense, odorless, colorless, and highly radioactive gas. Its radioactivity, ubiquity, and tendency to accumulate in homes make it a health hazard and a major contributor to lung cancer.
History
In 1785, Henry Cavendish found that a very small portion (less than 1/120) of the air could not be reacted in the experiments that reacted oxygen and nitrogen. This clue was not followed, however, and it was 1882 before a was discovered by Lord Rayleigh and Sir William Ramsay. In experiments to measure the of gases, Rayleigh found that the density of nitrogen from ammonia and that from air with the oxygen removed were not the same. Ramsay then studied atmospheric nitrogen. By reacting the nitrogen with red-hot magnesium, he isolated a small amount of much denser gas. When its spectrum was examined, there were lines that did not match any known element. This new was named argon, from the Greek word for idle or lazy, because of its inert nature.
Ramsay suspected that another element might exist between argon and helium (which had been discovered in the Sun in 1868), as their atomic weights of 40 and 4 were so different. In May 1898, Ramsay and Morris William Travers allowed liquid air to boil away gradually until only a small amount was left. They removed the nitrogen and oxygen with red-hot copper and magnesium. When they examined the spectrum, there were new lines. This new element was named krypton, from the Greek word for hidden.
Krypton was a new element of the group, but it was not the one for which they had searched. In June 1898, Ramsay and Travers liquefied and solidified an argon sample. Instead of keeping the last gas to boil away (which had led them to krypton), they kept the first fraction. When they examined the spectrum it produced, they found a blaze of crimson light unlike that of any other element. The new element was named neon, from the Greek word for new.
Ramsay and Travers continued their search for elements using a new liquid-air machine supplied by Ludwig Mond. By repeated fractionation of krypton, a still heavier gas was extracted in July 1898. The spectrum identified it as a new element, which they called xenon, from the Greek word for stranger. It has been known for some time that clathrates, organic hydroxy compounds with large cavities, would contain (but not bond to) the larger noble gases (argon, krypton, and xenon), but it was not until 1962 that compounds of the noble gases were first made by Neil Bartlett. Most of the compounds are xenon, but a few are krypton with fluorine or oxygen. No compounds of neon or argon have been prepared.
Obtaining the Noble Gases
The noble gases are obtained as a by-product of the liquefaction and separation of air. Dry carbon-dioxide-free air is liquefied and distilled. The volatile fraction contains nitrogen, neon, and helium. The remaining liquid of oxygen, argon, krypton, and xenon is fractionated to yield argon contaminated with oxygen. The oxygen is removed by reaction with hot copper-copper oxide. Further separation of the gases is achieved by selective adsorption and desorption with charcoal. Some argon is obtained as a by-product in the production of ammonia (NH3). The argon is an impurity in the nitrogen and hydrogen gases. About 635,000 metric tons of argon are obtained annually. Smaller amounts of the other gases are collected.
Uses of Noble Gases
The main use of argon is as an inert atmosphere for high-temperature metallurgical work. It is also used to fill incandescent lamps. The inert atmosphere allows the filament to burn for a long period of time before it burns out. Argon is also used in lasers and Geiger counters (radiation counters). The naturally occurring presence of argon isotopes is used to date geological formations. There are two methods that use the amount of argon isotopes to date materials in the millions of years range. One method uses the argon 40 to argon 39 ratio; the other uses the argon 40 to potassium 40 ratio.
All the noble gases are used in discharge tubes (neon lights). Each gas produces a particular color—for example, red by neon and blue by xenon. Other colors can be produced by a combination of gases. The neon-light industry was started by Georges Claude in the early 1900s and grew into a large industry. Fluorescent tubes are also filled with the noble gases, but the color of the tube depends on the phosphor coat on the inside of the tube. The denser noble gases, especially argon, have been used to fill the space between layers of glass in thermal insulating windows. Neon is also used in fog lights, television tubes, lasers, and voltage detectors. Krypton is used in flashbulbs and ultraviolet lasers. The wavelength of one isotope of krypton is the standard for the metric system. Xenon is also used in ultraviolet lamps, sunlamps, paint testers, projection lamps, and electronic flashes. Radon has been used in radiation therapy to treat cancers but, for the most part, has been superseded by radionuclides. It also has some uses in research.
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