Krypton - Kr
Krypton (Kr) is a trace atmospheric noble gas used in specialised lighting and scientific applications. Its isotopes also help investigate groundwater. This guide distinguishes ordinary krypton gas from radioactive krypton and explains the associated handling considerations.
Chemical properties · Uses and occurrence · Health effects · Environmental effects · Water and treatment
Chemical properties of krypton
| Symbol / atomic number | Kr / 36 |
|---|---|
| Group / period / block | 18 / 4 / p |
| Standard atomic weight | 83.798(2) |
| Electron configuration | [Ar] 3d10 4s2 4p6 |
| State at 20 °C and 1 bar | Gas |
| Gas density (ideal, 20 °C, 1 bar) | 3.44 g/L |
| Melting point | −157.37 °C |
| Boiling point | −153.415 °C |
| Non-bonded atomic radius | 0.202 nm |
| First ionisation energy | 1350.8 kJ mol−1 |
| Principal naturally occurring isotopes | 78Kr, 80Kr, 82Kr, 83Kr, 84Kr, 86Kr |
| Discovery | William Ramsay and Morris Travers, 1898 |
| CAS registry number | 7439-90-9 |
Atomic weight follows CIAAW; parentheses indicate uncertainty in the final digits. Physical properties follow the Royal Society of Chemistry unless noted. Atomic radii depend on their definition. Gas density is an ideal-gas estimate from ρ = pM/(RT), using CIAAW atomic weight and the stated temperature and pressure.
Uses, occurrence and history
Krypton is a colourless, odourless noble gas. It is chemically unreactive under ordinary conditions, although compounds such as krypton difluoride (KrF2) can be made under specialised conditions. It is a nonmetal, not a metal.
It occurs at approximately one part per million by volume in air and is recovered by separating liquefied air. Applications include specialised lamps and certain laser systems. William Ramsay and Morris Travers discovered krypton in 1898 while examining material separated from liquid air.
From 1960 to 1983, the metre was defined using a spectral line of krypton-86. The definition used exactly 1,650,763.73 vacuum wavelengths of that radiation. The modern metre is defined through the fixed speed of light; the krypton standard is historical.
Health effects and handling
Krypton has no established biological role. Its main hazard as an ordinary compressed gas is oxygen displacement: a leak can create an oxygen-deficient atmosphere and cause rapid suffocation. The absence of colour or smell gives no reliable warning. Gas concentration alone should not be used as a substitute for measuring oxygen and assessing ventilation.
Compressed cylinders also present pressure hazards. Use equipment and handling procedures appropriate to the gas supply. A liquefied gas presents additional cold-contact hazards, distinct from those of gas at room temperature. Do not enter a suspected oxygen-deficient space without appropriate assessment and controls.
Radioactive krypton is a separate consideration. Krypton-85 is radioactive, with a half-life of approximately 11 years; it must not be described as a stable isotope. Radiological assessment depends on the isotope, activity and exposure conditions.
Krypton in the environment
Natural atmospheric krypton is mainly composed of the isotopes listed above. Very small amounts of radioactive krypton isotopes also occur in the atmosphere and hydrosphere. Krypton-81 is produced naturally by atmospheric processes, while nuclear activities can release krypton-85.
Chemical inertness does not remove the need to consider oxygen deficiency near a large release, or radiation where radionuclides are involved. Routine compressed-gas handling and radioactive-material management are different tasks.
Krypton in water and groundwater studies
Dissolved krypton isotopes are useful environmental tracers. Krypton-81 can be used to investigate the age of old groundwater, helping researchers assess aquifer residence times and groundwater resources. Such measurements require specialised sampling and analysis.
For a water investigation, distinguish stable dissolved gas from a radioactive isotope and report the appropriate concentration or activity. A tracer-derived groundwater age describes water history; it does not by itself establish drinking-water quality.
Krypton is not a conventional water disinfectant or a substitute for treatment of dissolved salts and other contaminants. Its principal relevance here is groundwater characterisation rather than a routine purification step.
Krypton page last reviewed: by Lenntech.
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