Atomic number 54

Xenon

Xe
+ phase
− phase

Shells Orbitals 4d 5s 5p

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54 electrons in 5 shells: 2, 8, 18, 18, 8.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Noble gas
Period: 5
Group: 18
Block: p
Standard atomic weight: 131.29(1) Da
Electron configuration: [Kr] 4d¹⁰ 5s² 5p⁶
Phase at 25 °C: gas
Melting point: −111.75 °C
Boiling point: −108.099 °C
Density: 5.887 g/L
Electronegativity (Pauling): 2.6
Discovered: 1898, William Ramsay and Morris Travers, in the residue of evaporated liquid air

Neighbours in the table

I Cs

The name comes from the Greek xenos, stranger. William Ramsay and Morris Travers found it in 1898 in the residue left once liquid air had boiled away, soon after their discovery of krypton and neon. Jefferson Lab’s history in PubChem’s record dates the find to 12 July 1898.

For six decades chemists assumed that xenon formed no compounds. Group 18 had been called noble because its members were thought not to react. Neil Bartlett overturned that assumption in 1962, working at the University of British Columbia. Los Alamos now counts more than 80 compounds with xenon bonded to fluorine and oxygen, and OpenStax calls xenon the most reactive of the noble gases.

Its fluorides count all eight outer electrons, the 5s and 5p set, around the xenon atom. Xenon difluoride, XeF₂, forms colourless crystals that keep at room temperature in dry air. OpenStax’s Lewis structure gives its xenon two bonding pairs and three lone pairs. These five pairs point to the corners of a trigonal bipyramid, two triangular pyramids joined base to base, yet the molecule is linear. In XeF₄ the xenon atom carries four bonds and two lone pairs, which sit on opposite sides to keep their repulsion low. That leaves the four fluorines in a square plane.

Where it is found

Los Alamos puts xenon at about one part in twenty million of the atmosphere, while Jefferson Lab gives a higher 0.0000087 per cent. Gases escaping from certain mineral springs carry some, and the Martian atmosphere holds 0.08 parts per million. Commercial xenon comes from liquid air by fractional distillation, the same process that yields argon, neon and krypton.

Natural xenon has nine stable isotopes according to Los Alamos, which also counts 20 unstable ones. Jefferson Lab’s count in PubChem’s record gives six instead. IAEA nuclear data in that record give xenon-136 a half-life of 2.18 zettayears, 2.18 × 10²¹ years, by double beta decay.

A share of the xenon-129 on Earth is radiogenic, the product of iodine-129 decaying with a half-life of 1.7 × 10⁷ years. Against the age of the Earth that is brief, so the primordial iodine has all but vanished and its daughter xenon is the trace it left. That primordial iodine came largely from supernovae. Radiogenic xenon-129 in meteorites and terrestrial samples implies that the planets condensed a short time after the iodine was forged, short compared with the Solar System’s later history.

Why it matters

An electric discharge turns xenon into light. In a vacuum tube it glows blue, and in a strobe lamp it gives a brilliant white flash. Los Alamos lists electron tubes, stroboscopic and bactericidal lamps, the lamps that excite ruby lasers and very bright lamps built for observing the deep sea.

Xenon-124 is the raw material for iodine-123 and iodine-125, isotopes used in diagnosis and in cancer treatment respectively. The perxenates, compounds of the perxenate ion XeO₆⁴⁻ in which xenon reaches an oxidation state of +8, serve analytical chemists as oxidising agents. An oxidation state is a bookkeeping number that treats every bond as if it were ionic. Two radioactive isotopes, xenon-133 and xenon-135, come from neutron irradiation, Los Alamos notes, in air-cooled nuclear reactors, and the same source says that xenon-133 has useful applications of its own.

In medicine, xenon is breathed in. IUPAC’s notes in PubChem’s element record explain that inhaled xenon-127 and xenon-133, along with a specially prepared form of xenon-129, can be tracked through the body. The resulting images show how well the blood takes up oxygen and carries it onward.

Properties

Properties of xenon Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	54
Standard atomic weight	131.29(1) Da	Da	IUPAC/CIAAW 2024
Melting point	−111.75 °C	°C	Los Alamos: −111.75 °C
Boiling point	−108.099 °C	°C	Los Alamos: −108.099 °C
Density	5.887 g/L	g/L
Electronegativity (Pauling)	2.6
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶
Phase at 25 °C	gas

Safety

Los Alamos describes elemental xenon as not toxic, and in the air or inside a sealed lamp it is essentially harmless. PubChem’s safety summary for xenon gives the signal word Warning for the container, not the gas: it may be under pressure, and the refrigerated liquid can cause cold burns. Xenon is heavier than air, so a large release can collect in low or closed spaces and leave too little oxygen there. The PubChem Laboratory Chemical Safety Summary is the sheet for those who store and handle it.

Questions

Why is xenon used in camera flashes?

Excited by an electric current, xenon gives off a brilliant white flash, and strobe lights rely on it widely. OpenStax describes krypton-xenon flash tubes used to take high-speed photographs, with a burst of light that lasts only about 1/50,000 of a second. Los Alamos adds stroboscopic lamps and the lamps that pump ruby lasers to the list of xenon's jobs in lighting.

How is xenon used to detect nuclear tests?

Xenon-131, xenon-133 and xenon-135 are among the fission products of uranium-235 and plutonium-239. IUPAC's notes in PubChem's element record explain that finding these isotopes points to a nuclear bomb reaction, so measuring them helps monitor compliance with nuclear test bans. The same measurements can reveal faults in nuclear reactors. IAEA data in the same record date the discovery of xenon-133 and xenon-135 to 1940.

Why did Deep Space 1 use xenon as fuel?

The space probe Deep Space 1 carried an experimental ion engine whose fuel was xenon ions. The Jefferson Lab history reproduced in PubChem's element record names one reason, xenon's high atomic weight; its standard atomic weight is 131.29 Da. Xenon also has uses in nuclear energy work, Los Alamos says.

Is xenon gas dangerous to breathe?

Los Alamos describes xenon itself as not toxic. At high concentrations, of the order of one atmosphere, Haz-Map notes that it acts as an anaesthetic, the effect called inert gas narcosis in divers who breathe mixed gases deep under the sea. A large leak into a low or closed space can also leave too little oxygen to breathe. The PubChem Laboratory Chemical Safety Summary is the reference sheet.

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