Xenon

Xenon is a heavy noble gas that glows brilliant white in light bulbs, powers the thrusters of spacecraft travelling to distant planets, and can even act as a general anaesthetic, an unusual property for a gas that forms almost no chemical compounds. It was named "the stranger" when discovered, because it was so hard to find.

• Atomic Number5454 protons, 54 electrons
• Atomic Mass131.29 u54× heavier than hydrogen
• State at Room TempGasGas
• Density0.005887 g/cm³
• Melting / Boiling-111.8°C / -108.1°C
• Discovered1898

What is Xenon?

Xenon is a noble gas in Group 18 of the periodic table. With 54 protons and a completely filled outer electron shell, it is essentially non-reactive under normal conditions, but unlike helium, neon and argon, it can be forced to form compounds with the most reactive elements, particularly fluorine. The first confirmed noble gas compound ever made (in 1962) was a xenon compound. Xenon is monatomic, colourless and odourless as a gas.

Xenon gets its name from the ancient Greek word xenos meaning stranger or foreign, chosen by William Ramsay and Morris Travers who discovered it in 1898, because it was so rare and difficult to isolate from liquefied air. The symbol Xe comes from the name. Its discovery came just weeks after neon and krypton, in an extraordinary period of noble gas discovery.

Fact Xenon ion thrusters are used to move spacecraft. Xenon atoms are ionised (stripped of an electron) and then accelerated by electric fields to very high speeds, shooting out the back of the spacecraft and propelling it forward. Ion thrusters provide 10 times less thrust than chemical rockets but use fuel 10 times more efficiently, making them ideal for long missions where fuel economy matters more than immediate speed.

Where you find Xenon

In space

Xenon is rare in the Earth's atmosphere but more abundant in the Sun and in the gas giant planets. Jupiter's atmosphere contains significantly more xenon relative to hydrogen than the Sun does, a puzzle that remains unsolved. Xenon-based ion thrusters propel some spacecraft.

On Earth

Xenon makes up only 0.0000087% of the atmosphere: one of the rarest stable gases in air.

• Industrial air separation. All xenon is extracted from the atmosphere by fractional distillation of liquid air. Production is only a few hundred tonnes per year worldwide.
• Noble gas abundance. Xenon is far rarer than krypton or argon in the atmosphere, making it one of the most expensive industrial gases.

How we use Xenon

• High-intensity lighting.. Xenon arc lamps produce an intensely bright, sun-like white light and are used in cinema projectors, searchlights, lighthouses and some car headlights (HID lamps).
• Ion thrusters.. Xenon ion propulsion systems are used on spacecraft such as the Dawn mission (which orbited Vesta and Ceres) and the Starlink satellites. Xenon is ionised and accelerated electrically to generate thrust with extraordinary efficiency.
• Medical anaesthesia.. Xenon can be used as a general anaesthetic, it induces unconsciousness with rapid onset and fast recovery, and unlike many other anaesthetics, has no harmful effects on the heart. It is expensive but used in some specialised procedures.
• Medical imaging.. Inhaled xenon-133 gas is used to image lung ventilation in nuclear medicine scans.

Did you know? Xenon was the subject of the Nobel Prize in Chemistry in 1962, not for xenon itself, but for the first compound ever made from any noble gas. Neil Bartlett had reacted xenon with the powerful oxidiser platinum hexafluoride to produce the first noble gas compound. This demolished the belief that noble gases were completely inert and opened up a new branch of chemistry.

How it was discovered

Xenon was discovered in 1898 by William Ramsay and Morris Travers in London, using the same technique they used for neon and krypton, careful evaporation of liquefied argon and collection of heavier residues. Xenon was the last atmospheric noble gas they discovered. Despite its heaviness and rarity in air, Ramsay and Travers isolated it using spectroscopy to confirm the distinctive xenon spectral lines.

Deeper dive: xenon chemistry and applications

Xenon difluoride (XeF₂), tetrafluoride (XeF₄) and hexafluoride (XeF₆) are the most important xenon compounds. They are oxidising agents that react with many substances but are stable enough to be stored in dry conditions. XeF₂ is used as a selective fluorinating agent in organic synthesis and in the semiconductor industry to etch silicon. Xenon oxides, XeO₃ and XeO₄, are explosive.

The anaesthetic properties of xenon are unusual among noble gases. Xenon molecules are thought to bind to the NMDA receptor, a key receptor in the nervous system, and block its action, producing unconsciousness. This is distinct from most other anaesthetics. Xenon anaesthesia has several advantages: rapid onset and recovery, no liver or kidney damage, minimal cardiac effects, and it is not a greenhouse gas (unlike some fluorinated anaesthetics). The main disadvantage is cost, xenon is far more expensive than conventional anaesthetic gases.

Moving to 55 protons on the periodic table brings us to Caesium.