where did the name noble gases come from
Noble gas Noble gas is translated from the German noun Edelgas, first used in 1898 by Hugo Erdmann[1] 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.[2] Rare gases is another term that was used,[3] but this is also inaccurate because argon forms a fairly considerable part (0.94% by volume, 1.3% by mass) of the Earth's atmosphere due to decay of radioactive potassium-40.[4]
Planetary nebula Physicists showed in the 1920s that in gas at extremely low densities, electrons can occupy excited metastable energy levels in atoms and ions that would otherwise be de-excited by collisions that would occur at higher densities.[10] Electron transitions from these levels in nitrogen and oxygen ions (O+, O2+ (a.k.a. O iii), and N+) give rise to the 500.7 nm emission line and others.[6] These spectral lines, which can only be seen in very low density gases, are called forbidden lines. Spectroscopic observations thus showed that nebulae were made of extremely rarefied gas.[11]