Xenon

Xenon is a dense, colourless noble gas in group 18 with the configuration [Kr] 4d10 5s2 5p6. Like all noble gases, its outer shell is full, but xenon's outermost electrons are far from the nucleus and well shielded, so its first ionization energy is relatively low. That makes xenon the noble gas most willing to react. With highly electronegative partners such as fluorine and oxygen, it forms real compounds, including xenon difluoride, xenon tetrafluoride and the explosive xenon trioxide, with xenon in oxidation states up to +8.

Xenon is rare, about 87 parts per billion of air, and is produced by fractional distillation of liquid air. Passing an electric discharge through it gives an intense white light close to daylight, used in camera flashes, cinema projectors and car headlights. Its heavy atoms are easy to ionize and carry a lot of momentum when accelerated, making xenon the standard propellant for ion thrusters. Xenon is also an effective general anaesthetic with few side effects, though cost limits its use. Tonnes of ultrapure liquid xenon fill underground detectors searching for dark matter.

Discovery

In July 1898 William Ramsay and Morris Travers at University College London were repeatedly distilling liquefied air. After finding krypton and neon, they isolated a heavier gas from the residue that glowed blue in a discharge tube. They named it xenon, from the Greek xenos, meaning 'stranger'. For decades noble gases were thought to be inert, until in 1962 Neil Bartlett at the University of British Columbia made a xenon compound with platinum hexafluoride.