Xenon
Xenon is a chemical element with symbol Xe and atomic number 54. It is a colorless, dense, odorless noble gas, that occurs in the Earth's atmosphere in trace amounts. Although generally unreactive, xenon can undergo a few chemical reactions such as the formation of xenon hexafluoroplatinate, the first noble gas compound to be synthesized.
At a glance
- Atomic number
- 54
- Atomic mass
- 131.2936 u
- Phase
- Gas
- Density
- 5.894 g/L
- Melting point
- 161.4 K
- Boiling point
- 165.051 K
- First ionization energy
- 1,170.4 kJ/mol
- Electronegativity
- 2.6
- Electron affinity
- -77 kJ/mol
- Molar heat capacity
- Unavailable
- Period
- 5
- Appearance
- colorless gas, exhibiting a blue glow when placed in a high voltage electric field
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Xenon’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.
36 is the [Kr] core total across four shells.
Preparing the orbital view…
54 protons78 neutrons
Xenon-132: 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, xenon 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
XeNeutral atom · no charge
A neutral xenon atom. Removing its first electron takes 1,170 kJ/mol.
Xenon’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 1.7× 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 18, then 8, then 2 bars. Read backwards, that’s 2, 8, 18, 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 xenon’s valence electrons: a full outer shell. That’s why xenon sits in group 18 with the other noble gases, which all have full outer shells.
- Even the first electron is expensive: 1,170 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 | 5 | 1,170 | – | Measured |
| 2 | 5 | 2,024 | 1.7 | Estimated |
| 3 | 5 | 2,996 | 1.5 | Estimated |
| 4 | 5 | 4,072 | 1.4 | Measured |
| 5 | 5 | 5,220 | 1.3 | Estimated |
| 6 | 5 | 6,436 | 1.2 | Measured |
| 7 | 5 | 8,838 | 1.4 | Estimated |
| 8 | 5 | 10,225 | 1.2 | Measured |
| 9 | 4 | 17,352 | 1.7 | Measured |
| 10 | 4 | 19,490 | 1.1 | Calculated |
| 11 | 4 | 22,097 | 1.1 | Estimated |
| 12 | 4 | 24,604 | 1.1 | Calculated |
| 13 | 4 | 27,112 | 1.1 | Calculated |
| 14 | 4 | 30,296 | 1.1 | Calculated |
| 15 | 4 | 33,094 | 1.1 | Calculated |
| 16 | 4 | 36,086 | 1.1 | Calculated |
| 17 | 4 | 38,980 | 1.1 | Calculated |
| 18 | 4 | 41,875 | 1.1 | Calculated |
| 19 | 4 | 52,970 | 1.3 | Calculated |
| 20 | 4 | 56,154 | 1.1 | Calculated |
| 21 | 4 | 59,435 | 1.1 | Calculated |
| 22 | 4 | 62,715 | 1.1 | Calculated |
| 23 | 4 | 67,540 | 1.1 | Calculated |
| 24 | 4 | 71,013 | 1.1 | Calculated |
| 25 | 4 | 78,925 | 1.1 | Calculated |
| 26 | 4 | 82,688 | 1 | Estimated |
| 27 | 3 | 144,053 | 1.7 | Calculated |
| 28 | 3 | 151,578 | 1.1 | Calculated |
| 29 | 3 | 159,490 | 1.1 | Calculated |
| 30 | 3 | 168,077 | 1.1 | Calculated |
| 31 | 3 | 176,182 | 1 | Calculated |
| 32 | 3 | 185,155 | 1.1 | Calculated |
| 33 | 3 | 195,190 | 1.1 | Calculated |
| 34 | 3 | 203,874 | 1 | Calculated |
| 35 | 3 | 213,136 | 1 | Calculated |
| 36 | 3 | 221,916 | 1 | Calculated |
| 37 | 3 | 246,617 | 1.1 | Calculated |
| 38 | 3 | 254,432 | 1 | Calculated |
| 39 | 3 | 263,019 | 1 | Calculated |
| 40 | 3 | 271,220 | 1 | Calculated |
| 41 | 3 | 287,044 | 1.1 | Calculated |
| 42 | 3 | 296,017 | 1 | Calculated |
| 43 | 3 | 312,902 | 1.1 | Calculated |
| 44 | 3 | 321,663 | 1 | Estimated |
| 45 | 2 | 739,078 | 2.3 | Calculated |
| 46 | 2 | 761,173 | 1 | Calculated |
| 47 | 2 | 785,777 | 1 | Calculated |
| 48 | 2 | 808,740 | 1 | Calculated |
| 49 | 2 | 865,570 | 1.1 | Calculated |
| 50 | 2 | 891,814 | 1 | Calculated |
| 51 | 2 | 924,426 | 1 | Calculated |
| 52 | 2 | 946,557 | 1 | Calculated |
| 53 | 1 | 3,885,631 | 4.1 | Calculated |
| 54 | 1 | 3,984,834 | 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
Xenon glows blue-violet. Its bright, white flash is used in camera flashes and some car headlights.
Electrons race through the tube and crash into xenon 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.
473.4 nmblue
An electron falls from the 6p level down to the 6s level. It loses 2.62 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 blue light. Bigger drops give bluer light.
The strongest visible lines of neutral xenon 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.
A full shell: nothing to give, no room to take
Xenon’s outer shell holds eight electrons: it’s full. That makes it one of the least reactive elements. Its neighbours in the table are the opposite: iodine is one electron short of a full shell, and caesium has one electron too many. Try taking an electron away from each, or giving each one more.
- Iodine7 outer electrons
- Xenon8 outer electrons · full shell
- Caesium1 outer electron
Blue dots are the outer electrons; the grey ones fill the shell underneath.
The noble gases: how hard each holds on
- Helium2,372 kJ/mol
- Neon2,081 kJ/mol
- Argon1,521 kJ/mol
- Krypton1,351 kJ/mol
- Xenon1,170 kJ/mol
- Radon1,037 kJ/mol
Going down the group, the outer shell sits further from the nucleus, so it’s held less tightly. Neon forms no compounds at all. Xenon’s grip is weak enough to be broken: in 1962 Neil Bartlett noticed that pulling an electron off xenon costs about the same as pulling one off an oxygen molecule, which he had already done. He tried it, and made the first compound of a noble gas.
Energies from the element data (NIST): the first ionization energy to pull an electron off, and the electron affinity released when one is added. Noble-gas atoms don’t hold an extra electron at all, so no value is shown for them. Shells are drawn flat and simplified.
How does it compare?
Density
0.00589 g/cm³10th lowest of 99
Melting point
161 K8th lowest of 96
Boiling point
165 K9th lowest of 97
Electronegativity
2.68th highest of 96
First ionization energy
1,170 kJ/mol10th 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.
- XenonRoyal Society of Chemistry
- XenonWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database
- Spectral linesNIST Atomic Spectra Database