Krypton
Krypton (from Greek:κρυπτός kryptos "the hidden one") is a chemical element with symbol Kr and atomic number 36. It is a member of group 18 (noble gases) elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquefied air, and is often used with other rare gases in fluorescent lamps.
At a glance
- Atomic number
- 36
- Atomic mass
- 83.7982 u
- Phase
- Gas
- Density
- 3.749 g/L
- Melting point
- 115.78 K
- Boiling point
- 119.93 K
- First ionization energy
- 1,350.8 kJ/mol
- Electronegativity
- 3
- Electron affinity
- -96 kJ/mol
- Molar heat capacity
- Unavailable
- Period
- 4
- Appearance
- colorless gas, exhibiting a whitish glow in a high electric field
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Krypton’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
18 is the [Ar] core total across three shells.
Preparing the orbital view…
36 protons48 neutrons
Krypton-84: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FGas
At this temperature, krypton is a gas.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
KrNeutral atom · no charge
A neutral krypton atom. Removing its first electron takes 1,351 kJ/mol.
Krypton’s outer shell holds 8 electrons. Electron 9 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 1.9× as much energy as the one before. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 8, then 18, then 8, then 2 bars. Read backwards, that’s 2, 8, 18, 8: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The 8 electrons before the first big jump are krypton’s valence electrons: a full outer shell. That’s why krypton sits in group 18 with the other noble gases, which all have full outer shells.
- Even the first electron is expensive: 1,351 kJ/mol. A full outer shell holds its electrons tightly, so noble gases have the highest first ionization energy in their row of the table, part of why they rarely react.
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 4 | 1,351 | – | Measured |
| 2 | 4 | 2,350 | 1.7 | Measured |
| 3 | 4 | 3,458 | 1.5 | Measured |
| 4 | 4 | 4,906 | 1.4 | Measured |
| 5 | 4 | 6,242 | 1.3 | Estimated |
| 6 | 4 | 7,573 | 1.2 | Estimated |
| 7 | 4 | 10,529 | 1.4 | Calculated |
| 8 | 4 | 12,138 | 1.2 | Measured |
| 9 | 3 | 22,481 | 1.9 | Estimated |
| 10 | 3 | 25,858 | 1.2 | Calculated |
| 11 | 3 | 29,717 | 1.1 | Calculated |
| 12 | 3 | 33,770 | 1.1 | Calculated |
| 13 | 3 | 37,726 | 1.1 | Calculated |
| 14 | 3 | 43,032 | 1.1 | Calculated |
| 15 | 3 | 47,471 | 1.1 | Calculated |
| 16 | 3 | 52,102 | 1.1 | Calculated |
| 17 | 3 | 57,023 | 1.1 | Calculated |
| 18 | 3 | 61,751 | 1.1 | Calculated |
| 19 | 3 | 75,771 | 1.2 | Estimated |
| 20 | 3 | 80,237 | 1.1 | Estimated |
| 21 | 3 | 85,177 | 1.1 | Estimated |
| 22 | 3 | 91,179 | 1.1 | Estimated |
| 23 | 3 | 96,389 | 1.1 | Estimated |
| 24 | 3 | 100,538 | 1 | Estimated |
| 25 | 3 | 111,441 | 1.1 | Estimated |
| 26 | 3 | 116,287 | 1 | Estimated |
| 27 | 2 | 282,596 | 2.4 | Estimated |
| 28 | 2 | 296,403 | 1 | Estimated |
| 29 | 2 | 311,455 | 1.1 | Estimated |
| 30 | 2 | 326,120 | 1 | Estimated |
| 31 | 2 | 345,803 | 1.1 | Estimated |
| 32 | 2 | 362,013 | 1 | Estimated |
| 33 | 2 | 383,143 | 1.1 | Estimated |
| 34 | 2 | 396,466 | 1 | Calculated |
| 35 | 1 | 1,668,851 | 4.2 | Calculated |
| 36 | 1 | 1,730,584 | 1 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
A fingerprint made of light
Krypton glows a smoky white. From 1960 to 1983 the metre was defined using the wavelength of an orange-red line from krypton-86.
Electrons race through the tube and crash into krypton atoms, knocking the atoms’ own electrons up to higher energy levels. As those fall back, the atoms give off light only at the exact wavelengths below, so each gas glows its own colour.
The five gases’ visible lines side by side: each pattern is a fingerprint. Choose one to see it in detail below.
Click a line to see where it comes from.
587.1 nmyellow
An electron falls from the 5p level down to the 5s level. It loses 2.11 eV of energyAn electronvolt (eV) is a tiny unit of energy used for atoms: the energy one electron gains crossing 1 volt. A particle of visible light carries between about 1.7 eV (red) and 3.3 eV (violet). and gives it off as one particle of yellow light. Bigger drops give bluer light.
The strongest visible lines of neutral krypton from the NIST Atomic Spectra Database. Line brightness follows NIST’s relative intensities, which only compare lines of the same element. Colours are approximate, and lamp colours are as they look to the eye.
How does it compare?
Density
0.00375 g/cm³9th lowest of 99
Melting point
116 K7th lowest of 96
Boiling point
120 K8th lowest of 97
Electronegativity
35th highest of 96
First ionization energy
1,351 kJ/mol6th highest of 99
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- KryptonRoyal Society of Chemistry
- KryptonWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database
- Spectral linesNIST Atomic Spectra Database