Xenon Xe

Discovery

In 1898 William Ramsay and Morris Travers, having separated krypton from liquid air, fractionally distilled that krypton again and found a heavier component. Its spectrum showed new blue lines, and they announced a new element, the last of the noble gases they discovered that year.

For more than sixty years afterwards, the noble gases were thought to be completely unreactive. In 1962 Neil Bartlett at the University of British Columbia noticed that the powerful oxidiser platinum hexafluoride could strip an electron even from molecular oxygen. Since xenon's ionisation energy is similar, he tried the same reaction with xenon.

The result was an orange-yellow solid, the first noble gas compound. Xenon difluoride, xenon tetrafluoride and other compounds soon followed, and chemistry textbooks had to be rewritten.

Origin of the name

The name comes from the Greek xenos, "stranger" or "foreigner". The symbol is Xe.

Main uses

• Lighting: xenon arc lamps produce intense, near-daylight white light for cinema projectors, solar simulators, high-intensity-discharge car headlights and camera flashes.
• Space propulsion: heavy and easy to ionise, xenon is the propellant in many ion thrusters on satellites and probes, such as NASA's Dawn mission to the asteroid belt.
• Medicine: xenon is an effective general anaesthetic with few side effects, but its cost limits its use. Hyperpolarised xenon-129 is used in lung MRI research.
• Semiconductors and lasers: xenon chloride excimer lasers and chip etching processes use xenon.
• Fundamental physics: detectors filled with tonnes of liquid xenon search for dark matter particles.

Isotopes

Natural xenon has nine isotopes, the most abundant being xenon-132 and xenon-129. Xenon-124 and xenon-136 decay extremely slowly; the double electron capture of xenon-124 was directly observed by the XENON1T experiment in 2019, with a half-life of about 1.8 × 10²² years.

Radioactive xenon-135, with a half-life of about nine hours, absorbs neutrons voraciously and causes "xenon poisoning", which suppresses reactor power. Xenon-133 (half-life about five days) is used in lung ventilation scans.

In everyday life

Air contains only about 0.09 parts per million of xenon, so huge volumes must be processed in air separation plants to obtain small amounts, making it one of the most expensive noble gases.

It is non-toxic, but because it is much denser than air, a leak in a confined space can pool at floor level and displace oxygen. Breathing xenon makes the voice deeper, the opposite of helium, but it carries a risk of asphyxiation and should not be tried.