For a long time, scientists thought that noble gases could not form compounds because their valence electron shell had no space for more electrons. This means that they could not accept any more electrons, which made bonding impossible. However, in 1933, Linus Pauling said that heavy noble gases could react with fluorine or oxygen, as they are the most electronegative atoms. His prediction was correct, and xenon hexafluoride (XeF) was later discovered. Many more noble gas compounds have been found since then.