With their closed-shell electron design the noble gas elements of group 18 were long regarded as chemically inert. However, in 1962 Bartlett pinpoint that the ionization energy of xenon was same to that of O2, and by reaction with PtF6 attempted to build the compound analogous to [O2]+[PtF6]-. He collect a complex product having the ion [XeF]+ rather than the required Xe+. Many substance of xenon are now known, widely with O and F, and few of krypton.
The gases are not usually abundant on Earth, although argon (formed by the radioactive decay of 40K) built up helium (formed by radioactive decay of uranium and thorium and about 1 mol % of the atmosphere occurs in natural gas. Radon is radioactive, 222Rn have a half-life of 3.8 days also being produce by radioactive decay from 238U. The boiling points of the elements display the trend expected from van der Waals' forces, that of helium (4.2 K) being the lowest of any element. Helium is also single as it does not solidify except under pressure; the remaining substance form monatomic solids with close-packed structures. Liquid helium is required for maintaining very low temperatures, argon as an inert gas in different metallurgical processes, and all the substance in gas discharge tubes.