Krypton is the chemical element with symbol Kr and atomic number 36. It is a colourless, odourless noble gas in group 18 of the periodic table. Krypton occurs only in trace amounts in Earth’s atmosphere and is obtained commercially during the fractional distillation of liquid air.
Krypton quick facts
| Property | Value |
| Symbol | Kr |
| Atomic number | 36 |
| Relative atomic mass | 83.798 |
| Group | 18 |
| Period | 4 |
| Block | p-block |
| State at 20°C | Gas |
| Density | 0.003425 g/cm³ |
| Melting point | −157.37°C |
| Boiling point | −153.415°C |
| Electron configuration | [Ar] 3d¹⁰ 4s² 4p⁶ |
| Common oxidation state | +2 in rare compounds |
| CAS number | 7439-90-9 |
| Key natural isotope | ⁸⁴Kr |
Position and electron configuration
Krypton is in group 18, period 4 and the p-block. Its filled valence shell, represented by [Ar] 3d¹⁰ 4s² 4p⁶, makes it highly unreactive under ordinary conditions.
See the complete CleverlySmart periodic table.
Physical and chemical properties
Krypton is a monatomic noble gas with no colour or smell. It is denser than air and liquefies only at very low temperatures. Although once considered completely inert, krypton can form a small number of compounds under extreme conditions.
The best-known krypton compound is krypton difluoride (KrF₂), in which krypton has the +2 oxidation state. KrF₂ is a powerful oxidising and fluorinating agent and is much less stable than ordinary compounds of reactive elements.
Occurrence and production
Krypton makes up roughly one part per million of Earth’s atmosphere by volume. It is recovered as a minor product from large air-separation plants. Air is cooled until it liquefies, and its components are separated by fractional distillation; krypton tends to concentrate with xenon in the heavier noble-gas fraction.
Uses of krypton
- Lighting: krypton is used in some high-performance incandescent and discharge lamps.
- Flash lamps: krypton-filled lamps have been used in high-speed photography and specialised pulsed-light systems.
- Lasers: krypton fluoride excimer lasers can generate ultraviolet light for scientific and industrial applications.
- Insulated glazing: krypton can fill narrow spaces between high-performance window panes to reduce heat transfer.
- Scientific measurement: from 1960 to 1983, the metre was defined using a specific spectral line of krypton-86.
Krypton isotopes
Natural krypton contains several stable or extremely long-lived isotopes, including ⁷⁸Kr, ⁸⁰Kr, ⁸²Kr, ⁸³Kr, ⁸⁴Kr and ⁸⁶Kr. Krypton-84 is the most abundant. Radioactive krypton isotopes are also produced in nuclear fission; krypton-85 is used in certain industrial and environmental tracing applications.
For broader isotope data, see the CleverlySmart complete isotope table.
Biological role and safety
Krypton has no known biological role. It is chemically non-toxic, but like other inert gases it can displace oxygen in enclosed spaces and create an asphyxiation hazard. Liquid krypton is cryogenic and can cause severe cold burns.
Discovery and name
William Ramsay and Morris Travers discovered krypton in 1898 while examining the residue left after most components of liquid air had evaporated. The name comes from the Greek kryptos, meaning hidden.
Frequently asked questions
Is krypton radioactive?
Natural krypton is mostly stable. Some radioactive isotopes, such as krypton-85, are produced by nuclear fission.
Can krypton form compounds?
Yes, although only under unusual conditions. Krypton difluoride, KrF₂, is the best-known example.
Krypton spectroscopy, lasers and isotope science
Krypton emits characteristic spectral lines when electrically excited, making it useful in discharge lamps and spectroscopy. Krypton fluoride, KrF, is used in excimer lasers that generate intense ultraviolet light for research, materials processing and some semiconductor-manufacturing applications.
Krypton isotopes are also useful scientific tracers. Long-lived radioactive isotopes such as krypton-81 can help date very old groundwater and polar ice because atmospheric krypton dissolves into water and becomes trapped when ice forms.
Rare chemistry of a noble gas
Noble gases were once considered completely inert, but krypton can form compounds under strongly oxidising conditions. Krypton difluoride (KrF₂) is the best-known example and is a powerful fluorinating agent, although it is far less stable than ordinary halogen compounds.
Frequently asked questions
Why is krypton expensive?
It is present only in trace amounts in air and must be separated during large-scale cryogenic air processing.
Is krypton dangerous?
It is chemically non-toxic, but concentrated krypton can displace oxygen in enclosed spaces, and liquid krypton can cause severe cold burns.
