Answer by
Att4372 (1704)
The outer electron shell is completely full. There is no room for any more electrons, so the gas cannot form a compound. For Helium, it's 2 electrons. In Neon and Argon, it's 8. (Krypton, Xenon, and Radon are the others.) They are in the last column of the periodic table - the valence shell full column.