Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Krypton was one of three noble gases discovered in 1898 by Scottish chemist and physicist Sir William Ramsay (1852-1916) and English chemist Morris William Travers (1872-1961). Ramsay and Travers discovered the gases by allowing liquid air to evaporate. As it did so, each of the gases that make up normal air boiled off, one at a time. Three of those gases—krypton, xenon, and neon, were discovered for the first time this way.
The term noble gas refers to elements in Group 18 (VIIIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. These gases have been given the name "noble" because they act as if they are "too arrogant" to react with other elements. Until the 1960s, no compound of these gases was known. Since they are so inactive, they are also called the inert gases. Inert means inactive.
Group 18 (VIIIA)
Krypton has relatively few commercial uses. All of them involve lighting systems in one way or another.
Discovery and naming
By 1898, two members of the noble gas family had been discovered. They were helium (atomic number 2) and argon (atomic number 18). But no other elements in the family had been found. The periodic table contained empty boxes between helium and argon and below argon. The missing noble gases had atomic numbers 10, 36, 54, and 86. Chemists think of empty boxes in the periodic table as "elements waiting to be discovered."
Since the two known noble elements, helium and argon, are both gases, Ramsay and Travers hoped the missing elements were also gases. And if they were, they might be found in air. The problem was that air had already been carefully analyzed and found to be about 99.95 percent oxygen , nitrogen , and argon. Was it possible that the missing gases were in the last 0.05 percent of air?
To answer the question, the chemists worked not with air itself, but with liquid air. Air becomes liquid simply by cooling it far enough. The colder air becomes, the more gases within it turn into liquids. At -182.96°C (-297.33°F), oxygen changes from a gas into a liquid. At -195.79°C (-320.42°F), nitrogen changes from a gas into a liquid. And so on. Eventually, all the gases in air can be made to liquefy (change into a liquid).
But the reverse process also takes place. Suppose a container of liquid air holds 100 liters. The liquid air will warm up slowly. When its temperature reaches -195.79°C, liquid nitrogen changes back to a gas. Since about 78 percent of air is nitrogen, only 22 percent of the original liquid air (22 liters) will be left.
When the temperature reaches -182.96°C, oxygen changes from a liquid back to a gas. Since oxygen makes up 21 percent of air, another 21 percent (21 liters) of the liquid air will evaporate.
The work of Ramsay and Travers was very difficult, however, because the gases they were looking for are not abundant in air. Krypton, for example, makes up only about 0.000114 percent of air. For every 100 liters of liquid air, there would be only 0.00011, or about one-tenth of a milliliter of krypton. A tenth of a milliliter is about a drop. So Ramsay and Travers—although they didn't know it—were looking for one drop of krypton in 100 liters of liquid air!
Amazingly, they found it. The discovery of these three gases was a great credit to their skills as researchers. They suggested the name krypton for the new element. The name was taken from the Greek word kryptos for "hidden."
Krypton is a colorless, odorless gas. It has a boiling point of -152.9°C (-243.2°F) and a density of 3.64 grams per liter. That makes krypton about 2.8 times as dense as air.
"Look, up in the sky! It's a bird! It's a plane....
T he famous cartoon character Superman has many super powers. Everybody knows that. He's the Man of Steel. He has X-ray vision. His hearing is so good, he can tune in on one voice in a crowded city. And, of course: He's faster than a speeding bullet! More powerful than a locomotive! Able to leap tall buildings in a single bound!
But there's one substance that weakens Superman: kryptonite! If exposed to kryptonite. Superman experiences pain and loses his super powers. If exposed for too long, he can even die.
Kryptonite, of course, is purely fictional. Despite the similarity in names, kryptonite has nothing to do with element 36, krypton. According to cartoon legend, Superman came from the planet Krypton.
Kal-El, as he was originally known, was placed in a spaceship by his parents, moments before the planet exploded.
Unfortunately, as the young Superman blasted away from Krypton, a piece of kryptonite got stuck on the spaceship. The same terrible forces that caused the planet to explode, also had created the deadly kryptonite. And, as Superman would later find out, arch-villains always seem to get their hands on this green glowing rock!
Aside from the fictitious nature of kryptonite, there is another difference between it and krypton. Kryptonite is a rock—one that can cause great harm to, well, one person anyway. Krypton is an inert gas that has no effect on anything.
For many years, krypton was thought to be completely inert. Then, in the early 1960s, it was found to be possible to make certain compounds of the element. English chemist Neil Bartlett (1932-) found ways to combine noble gases with the most active element of all, fluorine. In 1963, the first krypton compounds were made—krypton difluoride (KrF2) and krypton tetrafluoride (KrF4). Other compounds of krypton have also been made since that time. However, these have no commercial uses. They are only laboratory curiosities.
Occurrence in nature
The abundance of krypton in the atmosphere is thought to be about 0.000108 to 0.000114 percent. The element is also formed in the Earth's crust when uranium and other radioactive elements break down. The amount in the Earth's crust is too small to estimate, however.
Six naturally occurring isotopes of krypton exist. They are krypton-78, krypton-80, krypton-82, krypton-83, krypton-84, and krypton-86. 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.
At least sixteen radioactive isotopes of krypton are known also. 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.
One radioactive isotope of krypton is used commercially, krypton-85. It can be combined with phosphors to produce materials that shine in the dark. A phosphor is a material that shines when struck by electrons. Radiation given off by krypton-85 strikes the phosphor. The phosphor then gives off light. The same isotope is also used for detecting leaks in a container. The radioactive gas is placed inside the container to be tested. Since the gas is inert, krypton will not react with anything else in the container. But if the container has a leak, some radioactive krypton-85 will escape. The isotope can be detected with special devices for detecting radiation.
Krypton-85 is also used to study the flow of blood in the human body. It is inhaled as a gas, and then absorbed by the blood. It travels through the bloodstream and the heart along with the blood. Its pathway can be followed by a technician who holds a detection device over the patient's body. The device shows where the radioactive material is going and how fast it is moving. A doctor can determine whether this behavior is normal or not.
How long is a meter?
T he meter is the standard unit of length in the metric system. It was first defined in 1791. As part of the great changes brought by the French Revolution, an entirely new system of measurement was created: the metric system.
At first, the meter was defined in a very simple way. It was the distance between two lines scratched into a metal bar kept outside Paris. For many years, that definition was satisfactory for most purposes. Of course, it created a problem. Suppose someone in the United States was in the business of making meter sticks. That person would have to travel to Paris to make a copy of the official meter. Then the copy would have to be used to make other copies. The chances for error in this process are tremendous.
In 1960, scientists had another idea. They suggested using light produced by hot krypton as the standard of length. Here is how that standard was developed:
When an element is heated, it absorbs energy from the heat. The atoms present in the element are in an "excited," or energetic, state. Atoms normally do not remain in an excited state very long. They give off the energy they just absorbed and return to their normal, "unexcited" state.
The energy they give off can take different forms. One of those forms is light.
The kind of light given off is different for each element and for each isotope. The light usually consists of a series of very bright lines called a spectrum. The number and color of the lines produced is specific to each element and isotope.
When one isotope of krypton, krypton-86, is heated, it gives off a very clear, distinct, bright line with a reddish-orange color. Scientists decided to define the meter in terms of that line. They said that a meter is 1,650,763.73 times the width of that line.
This standard had many advantages. For one thing, almost anyone anywhere could find the official length of a meter. All one needed was the equipment to heat a sample of krypton-86. Then one had to look for the reddish-orange line produced. The length of the meter, then, was 1,650,763.73 times the width of that line.
This definition for the meter lasted only until 1983. Scientists then decided to define a meter by how fast light travels in a vacuum. This system is even more exact than the one based on krypton-86.
Krypton is still obtained by allowing liquid air to evaporate.
The only commercial uses of krypton are in various kinds of lamps. When an electric current is passed through krypton gas, it gives off a very bright light. Perhaps the most common application of this principle is in airport runway lights. These lights are so bright that they can be seen even in foggy conditions for distances up to 300 meters (1,000 feet). The lights do not burn continuously. Instead, they send out very brief pulses of light. The pulses last no more than about 10 microseconds (10 millionths of a second). They flash on and off about 40 times per minute. Krypton is also used in slide and movie projectors.
Krypton gas is also used in making "neon" lights. Neon lights are colored lights often used in advertising. They are similar to fluorescent light bulbs. But they give off a colored light because of the gas they contain. Some neon lights do contain the gas neon, but others contain other noble gases. A neon light filled with krypton, for example, glows yellow.
Compounds of krypton have been prepared in the laboratory but do not exist in nature. The synthetic (artificial) compounds are used for research purposes only.
Although neon lights sometimes do include neon, krypton is often the gas used.
There is no evidence that krypton is harmful to humans, animals, or plants.
Krypton is chemical element number 36 on the periodic table of the elements. It belongs to the group of elements known as the noble gases. The other noble gases are helium, neon, argon, xenon, and radon. Under normal conditions, krypton is a colorless, tasteless, odorless gas. Its density at normal temperature and pressure is about 0.5 oz per gallon (3.7 g per liter), making it nearly three times heavier than air. At extremely low temperatures, krypton may exist as a liquid or a solid. The boiling point of krypton is -243.81° F (-153.23° C), and its freezing point is only slightly lower at -251.27° F (-157.37° C).
Natural krypton is a mixture of six stable isotopes. Isotopes are atoms which have the same number of protons but which have different numbers of neutrons. The number of protons (the atomic number) determines which element is present, while the total number of protons and neutrons determines the atomic weight of the atom. The isotopes of krypton all have 36 protons and are named for their atomic weights. Krypton-84, which has 48 neutrons, is the most common isotope and makes up 57% of natural krypton. The other stable isotopes of krypton are krypton-86 (50 neutrons, 17.3%); krypton-82 (46 neutrons, 11.6%); krypton-83 (47 neutrons, 11.5%); krypton-80 (44 neutrons, 2.25%); and krypton-78 (42 neutrons, 0.35%)
Krypton can also exist as an unstable, radioactive isotope. These isotopes are created during nuclear reactions. About 20 radioactive isotopes of krypton have been produced. All of these isotopes except krypton-85 are very unstable, with half-lives of a few hours or less. (The half-life of a radioactive substance is the time required for half of the atoms in a sample of the substance to undergo radioactive decay.) Krypton-85, which has 36 protons and 49 neutrons, is much more stable, with a half-life of 10.73 years.
Krypton is used with argon in fluorescent lights to improve their brightness and with nitrogen in incandescent lights to extend their lifetime. It is also used in flashbulbs to produce a very bright light for a very short period of time, for use in high-speed photography. Radioactive krypton-85 can be used to locate small flaws in metal surfaces. The gas tends to collect in these flaws and its radioactivity can be detected.
The noble gases were completely unknown to humanity until fairly recently. The first hint of their existence came in 1785, when the English chemist Henry Cavendish discovered that air contained a small amount of an unknown substance that was less reactive than nitrogen. Nothing else was known about this substance until the late nineteenth century.
Meanwhile, the British astronomer Joseph Norman Lockyer discovered a new element in 1868. By analyzing light from the sun, he detected an unknown element that he named helium, from the Greek word helios (sun). Helium was not known to exist on Earth for more than a quarter of a century.
In 1894, the English physicist Lord Rayleigh (John William Strutt) and the Scottish chemist William Ramsay discovered a difference in the density of nitrogen obtained from the air and nitrogen obtained from ammonia. They soon discovered that the atmospheric nitrogen was mixed with a small amount of an unknown substance. By using magnesium to absorb the nitrogen, they were able to isolate the substance, which they named argon, from the Greek word argos (inactive), because it did not react with other substances.
In 1895, Ramsay and his assistant Morris William Travers discovered that the mineral clevite released argon and helium when heated. This was the first time helium was detected on Earth. In 1898, Ramsay and Travers obtained three new elements from air, which had been cooled into a liquid. They named these elements krypton, from the Greek word kryptos (hidden); neon, from the Greek word neos (new); and xenon, from the Greek word xenos (strange).
In 1900, the German chemist Friedrich Dom noted that the radioactive element radium released helium and an unknown radioactive gas as it decayed. In 1910, Ramsay and his assistant Robert Whytlaw-Gray determined the density of this unknown gas and named it niton, from the Latin word nitere (to shine), because its radioactivity caused it to glow when cooled to a liquid. Niton, later known as radon, was the last noble gas to be discovered. In 1904, Ramsay was awarded the Nobel Prize in Chemistry for his research of noble gases.
The noble gases were formerly known as the rare gases or the inert gases. It was later shown that some were quite common and that some were not completely unreactive. Helium is the second most common element in the universe and argon makes up about 1% of Earth's atmosphere. In 1962, Neil Bartlett created xenon platinum hexafluoride, the first chemical compound of a noble gas. Compounds of radon were created in the same year and compounds of krypton in 1963. No longer thought of as rare or inert, these elements came to be known as the noble gases. Like the so-called noble metals (gold, silver, platinum, etc.), they did not react with oxygen.
Krypton played an important role in science from 1960-1983, when the length of the meter was defined as 1,650,763.73 times the wavelength of the orange-red light emitted by krypton-86. The meter was later defined in terms of the speed of light in a vacuum, but krypton continues to be used in scientific research.
Although traces of krypton are found in various minerals, the most important source of krypton is Earth's atmosphere. Air is also the most important source for the other noble gases, with the exception of helium (obtained from natural gas) and radon (obtained as a byproduct of the decay of radioactive elements). At sea level, dry air contains 78.08% nitrogen and 20.95% oxygen. It also contains 0.93% argon, 0.0018% neon, 0.00052% helium, 0.00011% krypton, and 0.0000087% xenon. Other components of dry air include carbon dioxide, hydrogen, methane, nitric oxide, and ozone.
Krypton can also be obtained from the fission of uranium, which occurs in nuclear power plants. Unlike air, which contains only the stable isotopes of krypton, this process produces both stable isotopes and radioactive isotopes of krypton.
Making liquid air
- 1 Air is first passed through filters to remove particulate matter such as dust. The clean air is then exposed to an alkali (a strongly basic substance), which removes water and carbon dioxide.
- 2 The clean, dry air is compressed under high pressure. Because compression raises the temperature of the air, it is then cooled by refrigeration.
- 3 The cooled, compressed air passes through coils winding through an empty chamber. A portion of the air, which is compressed to a pressure about two hundred times greater than normal, is allowed to expand into the chamber. This sudden expansion absorbs heat from the coils, cooling the compressed air. The process of compression and expansion is repeated until the air has been cooled to a temperature of about -321 F (-196° C), at which point most of the gases in the air are transformed into liquids.
Separating the gases
- 4 Gases with very low boiling points are not transformed into liquids and can be removed from the others directly. These gases include helium, hydrogen, and neon.
- 5 A process known as fractional distillation separates the various elements found in liquid air. This process relies on the fact that the different substances will be transformed from liquid to gas at different temperatures.
- 6 The liquid air is allowed to warm slowly. As the temperature increases the substances with the lowest boiling points become gases and can be removed from the remaining liquid. Argon, oxygen, and nitrogen are the first substances to be transformed into gases as the liquid air warms. Krypton and xenon have higher boiling points and remain in the liquid state when the other components of air have become gases.
Separating krypton from xenon
- 7 The liquid krypton and xenon are absorbed onto silica gel or onto activated charcoal. They are then once again subjected to fractional distillation. The liquid mixture is warmed slowly until the krypton is transformed into a gas. The xenon has a somewhat higher boiling point and remains behind as a liquid.
- 8 The krypton is purified by passing it over hot titanium metal. This substance tends to remove all elements except noble gases.
Separating the isotopes of krypton
- 9 For most purposes, the krypton is now ready to be packaged. For some scientific purposes, however, only one of the six stable isotopes of krypton is desired. To separate these isotopes, a process known as thermal diffusion is used. This process depends on the fact that the isotopes have slightly different densities.
- 10 The krypton gas is placed in a long vertical glass tube. A heated wire runs vertically through the center of this tube. The hot wire sets up a convection current within the tube. This current of hot air tends to carry the lighter isotopes to the top of the tube, where they can be removed.
Packaging and shipping
- 11 Krypton gas is packed in bulbs of a strong glass such as Pyrex at normal pressure or in steel canisters at high pressure. Because it is a highly unreactive substance, krypton is very safe. It is nontoxic, nonexplosive, and nonflammable, so it requires no unusual precautions during shipping.
The most important factor in the quality control of krypton production is ensuring that the final product contains only krypton. The process of fraction distillation has been developed to the point where it produces very pure products from air, including krypton.
Random samples of krypton are tested for purity by spectroscopic analysis. This process involves heating a substance until it emits light. The light then passes through a prism or a grating in order to produce a spectrum, in the same way that sunlight produces a rainbow. Spectroscopic analysis is particularly well suited to studying gases, because heated gases tend to produce sharp, bright lines on a spectrum of pure krypton, it is possible to tell if any impurities are present.
Krypton is only one of many valuable elements produced by the fractional distillation of liquid air. More than three-quarters of air is made up of nitrogen. Nitrogen is used to produce a wide variety of chemical compounds, particularly ammonia. Because it is much less reactive than oxygen, nitrogen is used to protect many substances from oxidation. Liquid nitrogen is used in freeze-drying and refrigeration.
About one-fifth of air consists of oxygen. The steel industry is the largest consumer of pure oxygen. Oxygen is used to remove excess carbon from steel in the form of carbon dioxide. Oxygen is also used to treat sewage and to incinerate solid waste. Liquid oxygen is used as rocket fuel.
The noble gases obtained from air other than krypton are argon, neon, and xenon. Argon is used in certain types of light bulbs. Passing an electric current through a glass tube containing neon under low pressure produces the familiar neon sign. Xenon is used in strobe lights to produce intense, short bursts of light.
The future production of krypton is likely to be influenced by the future of nuclear power production. Because krypton can be produced as a byproduct of nuclear fission, nuclear power plants may become an important source of krypton in the future. On the other hand, if nuclear fission is largely replaced by nuclear fusion or by other forms of energy production, krypton is likely to remain almost entirely a product of the atmosphere.
Where to Learn More
Asimov, Isaac. The Noble Gases. Basic Books, 1966.
Atkins, P.W. The Periodic Kingdom: A Journey Into the Land of the Chemical Elements. Basic Books, 1995.
Compressed Gas Association. Handbook of Compressed Gases. Van Nostrand Reinhold, 1990.
Krypton (from the Greek word kryptos, meaning "hidden"), is the second heaviest of the noble gases . It was discovered in 1898 by Sir William Ramsay and Morris Travers during their experiments with liquid air, air that has been liquefied by cooling. It has a concentration of 1.14 ppm by volume in Earth's atmosphere. It is present in the Sun and in the atmosphere of Mars.
At room temperature krypton is a colorless, odorless gas. Upon freezing it forms a white crystal with a face-centered cubic structure. In a vacuum discharge tube, it emits primarily a mixture of green and yellow light. During the late twentieth century the wavelength of light corresponding to krypton's 605.78-nanometer (2.4 × 10−5-inch) spectral line was the internationally adopted definition of the meter. Krypton gas is used in the manufacture of fluorescence lights and flashlamps used in high-speed photography.
Krypton is produced deep within stars during nucleosynthesis . It has six naturally occurring (i.e., stable) isotopes , the most abundant of which is krypton-84 (57%). Some long-lived radioactive isotopes exist as well. Two of them, krypton-85 (half-life = 10.7 y) and krypton-81 (half-life = 210,000 y) have been used to date well water. Radioactive krypton is produced in fission reactions of heavy elements. Thus, radioactive isotopes of krypton have always formed part of the natural radiation background of Earth's atmosphere.
Although a noble gas, krypton is not entirely unreactive. One krypton compound, krypton difluoride (KrF2), is commercially available in small quantities.
see also Gases; Noble Gases; Ramsay, William; Travers, Morris.
Almqvist, Ebbe (2003). History of Industrial Gases. New York: Kluwer Academic/Plenum Publishers.
Lide, David R., ed. (2003). The CRC Handbook of Chemistry and Physics, 84th edition. Boca Raton, FL: CRC Press.
kryp·ton / ˈkripˌtän/ • n. the chemical element of atomic number 36, a member of the noble gas series. It is obtained by distillation of liquid air and is used in some kinds of electric light. (Symbol: Kr)