Krypton (Kr)
Krypton (Kr) is a noble gas with atomic number 36 and a standard atomic weight of 83.798. At room temperature it is a gas that melts at -157 °C and boils at -153 °C.
Krypton key facts
Krypton at a glance
Identity
| Symbol | Kr |
|---|---|
| Atomic number (Z) | 36 |
| Atomic weight | 83.798 ± 0.002 (CIAAW 2024) |
| Category | Noble gas |
| Group | 18 — noble gases |
| Period | 4 |
| Block | p-block |
| CAS number | 7439-90-9 |
Atomic properties
| Electron configuration | [Ar] 3d10 4s2 4p6 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 |
|---|---|
| Electrons per shell | 2, 8, 18, 8 |
| Oxidation states | +2 Less common: +1 |
| Electronegativity (Pauling) | 3 |
| First ionisation energy | 14 eV (1,350.8 kJ/mol) 2nd: 24.36 eV · 3rd: 35.838 eV |
| Covalent radius | 117 pm |
| Van der Waals radius | 202 pm |
Physical properties
| State at 25 °C | Gas |
|---|---|
| Melting point | -157.4 °C · -251.3 °F · 115.78 K |
| Boiling point | -153.4 °C · -244.2 °F · 119.73 K |
| Density | 3.749 g/L (gas at 0 °C, 1 atm) |
| Specific heat capacity | 0.248 J/(g·K) |
| Thermal conductivity | 0.009 W/(m·K) |
| Crystal structure | Face-centred cubic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 84Kr 56.987%, 86Kr 17.279%, 82Kr 11.593%, 83Kr 11.5%, 80Kr 2.286%, 78Kr 0.355% |
| Stable isotopes | 6 |
| Known isotopes | 35 |
| Abundance in Earth's crust | 0.0001 mg/kg |
| Abundance in seawater | 0.00021 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Krypton electron configuration
Krypton has 36 electrons. Written in short form, its ground-state configuration is [Ar] 3d10 4s2 4p6 — the electrons of the noble gas argon, plus 3d10 4s2 4p6.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6.
As a member of group 18 it has 8 valence electrons in its outer shell (shell 4), which largely decide its chemistry — a full outer shell, which is why it is so unreactive.
Discovery and name
1898 — William Ramsay and Morris Travers · England.
Greek kryptos, hidden.
Known for
An inert gas used in energy-efficient lighting, photography flashes and some lasers. The metre was defined by a krypton-86 spectral line from 1960 to 1983.
Isotopes of krypton
Naturally occurring krypton is a mix of 6 isotopes. Each has 36 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 84Kr | 36 | 48 | 56.987% | stable |
| 86Kr | 36 | 50 | 17.279% | stable |
| 82Kr | 36 | 46 | 11.593% | stable |
| 83Kr | 36 | 47 | 11.5% | stable |
| 80Kr | 36 | 44 | 2.286% | stable |
| 78Kr | 36 | 42 | 0.355% | stable |
35 isotopes of krypton are known in our data set (half-lives as published in nuclear data tables).
Krypton compared with group 18
The other elements in group 18 (noble gases), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Helium | 2 | 4.0026 | — | -269 °C | 0.1786 g/L | — |
| Neon | 10 | 20.180 | -249 °C | -246 °C | 0.9002 g/L | — |
| Argon | 18 | 39.95 | -189 °C | -186 °C | 1.784 g/L | — |
| Krypton | 36 | 83.798 | -157 °C | -153 °C | 3.749 g/L | 3 |
| Xenon | 54 | 131.29 | -112 °C | -108 °C | 5.894 g/L | 2.6 |
| Radon | 86 | [222] | -71 °C | -62 °C | 9.73 g/L | 2.2 |
| Oganesson | 118 | [294] | — | — | 7 g/cm³ (pred.) | — |