Noble gas After the risks caused by the flammability of hydrogen became apparent, it was replaced with helium in blimps and balloons. "Noble gas" is translated from the German noun , first used in 1898 by Hugo Erdmann to indicate their extremely low level of reactivity. The name makes an analogy to the term "noble metals", which also have low reactivity. The noble gases have also been referred to as "inert gases", but this label is deprecated as many noble gas compounds are now known. "Rare gases" is another term that was used, but this is also inaccurate because argon forms a
Noble gas applied at a temperature of to convert it to a solid. The noble gases up to xenon have multiple stable isotopes. Radon has no stable isotopes; its longest-lived isotope, Rn, has a half-life of 3.8 days and decays to form helium and polonium, which ultimately decays to lead. Melting and boiling points generally increase going down the group. The noble gas atoms, like atoms in most groups, increase steadily in atomic radius from one period to the next due to the increasing number of electrons. The size of the atom is related to several properties. For example, the ionization potential