Krypton – facts and properties
- Symbol
- Kr
- Atomic number
- 36
- Atomic mass
- 83.798 u
- Period
- 4
- Group
- 18
- Category
- noble gas
- Electron configuration
- [Ar] 3d¹⁰ 4s² 4p⁶
- Electronegativity
- 3
- Atomic radius
- 88 pm
- Valence electrons
- 8
- State at standard conditions
- gas
- Melting point
- -157.36 °C
- Boiling point
- -153.22 °C
- Density
- 3.73 g/L
- Discovered
- 1898
Krypton is a colourless, odourless noble gas with atomic number 36. It is in the air around you, but only as a trace, which is why nobody found it until 1898. Its name means "the hidden one". Krypton is heavier than air and reacts with almost nothing, and that inertness is exactly what makes it useful: it fills windows and lamps, serves as the gas in lasers used for chip production and, for more than two decades, defined the length of the metre.
Position in the periodic table
Occurrence and extraction
Krypton is one of the rarest gases in Earth's atmosphere: out of a million parts of air, only about one part is krypton. It is not found in any ore, so all of it is obtained from air. The air is cooled until it becomes liquid and is then separated by fractional distillation. Krypton ends up in a fraction together with xenon, from which it is split off.
The element was discovered in 1898 in London by the British chemists William Ramsay and Morris Travers. They let liquid air evaporate slowly and examined the last residue. Its spectrum showed bright lines that matched no known substance. Ramsay had identified argon and helium a few years earlier and soon afterwards found neon and xenon as well. They named the new gas after the Greek kryptos, hidden, because it had evaded discovery for so long.
Small amounts of krypton also form in nuclear fission. Spent fuel contains radioactive krypton-85, which can escape during reprocessing and is measurable as a trace in the air.
Properties and reactions
Krypton is in period 4 and group 18, the noble gases. Its electron configuration is [Ar] 3d¹⁰ 4s² 4p⁶. The outer shell is full with 8 valence electrons, which explains why the gas hardly reacts. An electronegativity of 3 is quoted for it, but this matters little because krypton rarely forms bonds. As a gas it has a density of 3.73 g/L, about three times that of air. It liquefies at -153.22 °C and freezes at -157.36 °C. These two temperatures lie very close together, as they do for most noble gases.
For a long time the noble gases were thought to be completely unreactive. After xenon, krypton was the next noble gas to give a true compound. In the early 1960s chemists made krypton difluoride, KrF₂, from krypton and fluorine. It is a colourless solid that keeps only when cold and slowly decomposes at room temperature. Fluorine is the most reactive nonmetal and can pull electrons away from krypton. There are no stable everyday compounds.
In a gas-discharge tube krypton glows with a whitish to bluish light when high voltage is applied.
Krypton is not toxic, but in large amounts in a closed room it is dangerous, because as a heavy gas it displaces oxygen and can cause suffocation.
Uses in everyday life and technology
Most krypton is used as a filling gas. In double- and triple-glazed windows it partly replaces argon. Krypton conducts heat even less well, so a narrower gap between the panes is enough, which helps with slim window frames. In incandescent lamps, especially halogen lamps and torch bulbs, a krypton filling protects the filament. The heavy gas slows the evaporation of tungsten from the wire, so the lamp can burn brighter and longer. Fluorescent lamps and photographic flash lamps use krypton too.
An important role belongs to the excimer laser. Here krypton briefly forms an excited molecule with fluorine, which then gives off ultraviolet light. This light is used to expose a light-sensitive resist on silicon wafers, transferring extremely fine patterns onto the chip. Such lasers are used in the manufacture of microchips.
The history of the metre is a special story. From 1960 to 1983 the metre was defined through the light of the isotope krypton-86, more precisely through the wavelength of one orange-red spectral line. This was more accurate than the old platinum-iridium bar kept near Paris. Since 1983 the metre has been defined through the speed of light, and krypton has handed that job on.
Quick question
What is the atomic number of krypton (Kr)?
FAQ
Is krypton toxic?
No, krypton is almost completely unreactive and not poisonous. It becomes dangerous only in closed rooms with a great deal of gas, because it displaces oxygen. The radioactive isotope krypton-85 does give off radiation but occurs only in traces. No hazard is to be expected in everyday life or in school lessons.
What is krypton used for?
Mainly as a filling gas in windows and lamps, where it conducts heat poorly and protects filaments. It is also found in excimer lasers used to make microchips. In the past its light was used to define the metre.
How was krypton discovered?
In 1898 William Ramsay and Morris Travers let liquid air evaporate and studied the last residue. The spectrum showed lines that fitted no known element. They called it krypton, the hidden one, because it had escaped detection for so long.