Krypton (Kr)
Krypton (Kr) is a chemical element belonging to the noble gases, which include helium, neon, argon, xenon, and radon. Known for its very low reactivity, krypton was historically referred to as an inert gas, with only a few hundred compounds identified under extreme conditions. Discovered in 1898 by chemists William Ramsay and Morris M. Travers, krypton’s name derives from the Greek word meaning "the missing one." It is extremely rare on Earth, constituting about one part per million of the atmosphere, yet it is more abundant in space. Krypton appears as a colorless, odorless, and tasteless gas at standard conditions and has specific applications in lighting, such as in fluorescent bulbs and high-intensity lamps, where it produces a bright, white light. Additionally, krypton is utilized in laser technologies and plays a role in advanced scientific research, including particle physics and nuclear fusion studies. Despite its low toxicity, krypton can act as an asphyxiant at high concentrations by displacing oxygen. Its unique properties continue to make krypton a subject of interest in various scientific and industrial fields.
Krypton (Kr)
- Element Symbol: Kr
- Atomic Number: 36
- Atomic Mass: 83.8
- Group # in Periodic Table: 18
- Group Name: Noble gases
- Period in Periodic Table: 4
- Block of Periodic Table: p-block
- Discovered by: Sir William Ramsay, Morris W. Travers (1898)
Krypton is one of the noble gases, a group of elements that also includes helium, neon, argon, xenon, and radon. All of the noble gases are very chemically unreactive. In fact, at one time it was thought that krypton and the other noble gases could not react with any other elements at all. This is why they used to be known as the inert gases. Eventually, it was discovered that a few hundred compounds can be found in nature that are made from noble gases. These gases only react under extreme conditions. The name "noble" was borrowed from the noble metals that are also very nonreactive, such as silver, gold, and platinum. The noble gases were also known as the rare gases. Krypton is rare on Earth. Only one part per million of Earth’s atmosphere is made up of the element. Not all of the rare gases are actually so rare. One of the group, argon, makes up almost 1 percent of Earth’s atmosphere by mass.
In 1898, English chemist William Ramsay and Scottish chemist Morris M. Travers analyzed the residue in liquefied air and discovered krypton. Its name comes from Greek and means "the missing one." Further research by Ramsay led to a series of discoveries of other noble gases, including neon. He won the Noble Prize in Chemistry for this work in 1904.
In 1960 the International Conference on Weights and Measures was looking for a more precise way to define a meter than the distance between two marks on a platinum-iridium bar representing one-ten-millionth of a quadrant of Earth’s polar circumference. The organization decided that a meter measured 1,650,763.73 wavelengths of orange spectral light emitted by an isotope of krypton. In 1983 this definition changed again to the distance light travels in a vacuum in 1/299,792,458 second.
Physical Properties
Krypton is a colorless, odorless, and tasteless gas in its standard state—that is, its state at 298 kelvins (K). It has a density of 3.749 grams per liter (g/L). Its melting point is −157.37 degrees Celsius (°C), and its boiling point is −153.42 °C. If it is solidified, its crystals are white and cubic. The specific heat of an element is the amount of energy required to raise the temperature by 1 degree. For krypton, the specific heat is 248.05 joules per kilogram-kelvin (J/kg·K).
Chemical Properties
The reason krypton and the other noble gases are almost inert chemically is that their outermost, or valence, electron shells are full. For these atoms to react chemically, something has to first remove one or more of their valence electrons. No compound made from a noble gas was found before 1960. Compounds containing krypton and fluorine have since been synthesized using techniques such as electrical discharges, proton bombardment, ultraviolet irradiation, and heating with extremely hot wires. One of the compounds produced is called krypton difluoride (KrF2). It is one of the strongest oxidizing agents known.
Krypton is one of the elements produced during the fission of uranium and plutonium. There are six stable isotopes of krypton and about thirty unstable ones. One radioactive isotope has a half-life of 230,000 years and has been used to date groundwater. Another radioactive isotope has a half-life of 10.76 years. This isotope is released during the reprocessing of nuclear fuel rods for reactors and during nuclear bomb testing. It is also used to detect when countries are developing nuclear reactors or nuclear weapons.
Applications
Krypton makes up only 0.0001 percent of the air in Earth’s atmosphere. There is some uncertainty about how much of it is found in the universe, but some readings indicate that there may be more than expected. Krypton can be isolated by fractionally distilling liquefied air. This expensive process means that it can be very costly to use krypton in manufacturing. Because it is so nonreactive, krypton is often used in situations in which it is important to suppress chemical reactions.
When krypton gas is ionized, it emits multiple lines of light that make it appear as a bright, white light. It is, therefore, one of the gases used in incandescent bulbs to increase the usable life of filaments and to enable them to burn brighter. Like several of the noble gases, krypton is also used in fluorescent lighting, and it is often combined with argon to produce lamps that use much less energy. In fact many of the colorful neon signs used in advertising are made of tinted glass tubes filled with krypton because these kinds of bulbs make such a bright white light. Krypton has also been found to be very useful in conventional flash photography as well as in high-speed photography.
The powerful red spectral line of light that krypton emits is used in a variety of laser-based technologies, from laser light shows to DVD players. Krypton plays an important role in one of the most powerful lasers in the world. A krypton-fluorine laser is being used in research to develop a practical way to generate power using nuclear fusion. A tiny sphere of fuel is simultaneously zapped from all sides by an intense flash of laser light. That flash has five hundred times the power of the entire US electrical grid, but it only lasts for four-billionths of a second.
Krypton also plays a role in particle physics research. The CERN laboratory bordering Switzerland and France built a device that holds twenty-seven tons of krypton to research tiny particles called kaons, or k-mesons. This research may help explain why there is so much less antimatter in the universe than there is matter.
Krypton is also being used to develop safer ways to create images to study people’s airways using computed tomography (CT) scans and magnetic resonance imaging (MRI).
Krypton is considered a nontoxic asphyxiate. This means that at high concentrations it will not poison a person, but it could cause suffocation by replacing the oxygen in the blood.
Eugene R. McCormick
Bibliography
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