We continue to review some of the most important materials in heat treatment and metallurgy.
Krypton (chemical symbol: Kr)
Krypton is an odorless and colorless noble gas, meaning it is chemically inert except in rare cases. Krypton exists in the atmosphere at a concentration of about 0.0001% and costs 100 times more than argon, a more commonly used noble gas. For this reason, krypton’s uses are very limited. Once thought to be completely inert, krypton is now known to form several compounds. Krypton difluoride (KrF2), a volatile, colorless solid, is the simplest krypton compound to create.
As is the case with the other noble gases, krypton glows when exposed to an electrical charge (Fig. 1), forming plasma – an ionized gas considered to be the fourth state of matter. In 1960, the official length of a meter was defined by the 605 nm wavelength of the orange spectral line of krypton-86. The meter was later redefined in 1983 as the distance traveled by light in vacuum during a time interval of 1/299,792,458th of a second.
Krypton was discovered in 1898 in Great Britain by the combined effort of Scottish chemist Sir William Ramsay and English chemist Morris Travers. Ramsay (Fig. 2) had previously discovered helium and argon and was searching for other noble gases to fill the gaps in group zero of the periodic table. They chilled down air until it liquefied and then gradually heated it until the lighter gases evaporated away. The remaining gas was found to contain a mixture of oxygen and nitrogen (which they were able to remove), as well as argon and another unknown gas.
By way of spectral analysis they identified this new gas and recognized that it filled the empty spot on the periodic table below argon. Ramsay named the new gas after the Greek word “kryptos,” meaning hidden. He went on to discover all the remaining noble gases and was awarded the 1904 Nobel Prize in Chemistry "in recognition of his services in the discovery of the inert gaseous elements in air."
Krypton's white color when ionized makes it useful in photography for bulbs as a bright source of pure white light. It is also used to produce flashes for high-speed photography. Krypton gas is also combined with other gases to make luminous signs that glow with a bright greenish-yellow light.
Krypton (along with xenon) is used inside incandescent lamps to reduce filament evaporation and permit higher operating temperatures. Another use of krypton is in the krypton fluoride laser, which is used for photolithography and nuclear-fusion research. Atmospheric testing for krypton around North Korea and Pakistan in the early 2000s detected the presence of Krypton-85, which revealed the existence of secret nuclear fuel reprocessing facilities in those regions.
Here are a few important facts about krypton (Fig. 3).
- Atomic number: 36
- Atomic weight: 83.798
- Melting point: 115.79 K (-157.36°C or -251.25°F)
- Boiling point: 119.93 K (-153.22°C or -243.80°F)
- Density: 0.003733 grams per cubic centimeter
- Phase at room temperature: Gas
- Element classification: Nonmetal
- Period number: 4
- Group number: 18
- Group name: Noble gas