Xenon

Xenon was the last of the stable noble gases to be found, by Ramsay and Travers in 1898, and for sixty-four years the whole group was taught as chemically inert. Then in 1962 Neil Bartlett, having noticed that platinum hexafluoride could tear an electron out of oxygen and that xenon's ionisation energy is very close to O₂'s, mixed the two and got an orange-yellow solid — the first noble gas compound, and the end of a textbook doctrine. Xenon is a general anaesthetic with almost no cardiovascular depression, though its cost keeps it niche. Ion thrusters accelerate xenon ions to tens of kilometres per second and flew NASA's Dawn mission to Vesta and Ceres. And tonnes of liquid xenon sit underground in detectors like LZ and XENONnT, waiting for a dark matter particle to recoil off a nucleus.