Final answer:
The noble gases except helium have eight valence electrons in their outermost shell, following the octet rule for maximum stability, with helium being an exception possessing only two.
Step-by-step explanation:
The number of valence electrons in the outermost shell of all noble gases except helium is eight. Noble gases are located in Group 18 (or Group VIII) of the periodic table, and they are well known for their chemical inertness which primarily arises due to having full valence electron shells. Helium is an exception as it has a full outer shell of two electrons, rather than eight. All other noble gases, such as neon, argon, krypton, xenon, and radon, follow the octet rule with eight valence electrons in their outermost shell, granting them exceptional stability and making them the least reactive elements.