Answer
The only noble gas that does not have eight electrons in its highest occupied energy level is helium. Helium has only two electrons in its outermost energy level (and the only one). Helium, however, is still stable because the s sub-shell has only one orbital, and thus space for only two electrons. Since the orbital and the sub-shell are completely full, helium does not need to react with other substances to complete its valence shell.
Work Step by Step
The only noble gas that does not have eight electrons in its highest occupied energy level is helium. Helium has only two electrons in its outermost energy level (and the only one). Helium, however, is still stable because the s sub-shell has only one orbital, and thus space for only two electrons. Since the orbital and the sub-shell are completely full, helium does not need to react with other substances to complete its valence shell.
