Each noble gas has a full valence electron shell, which makes them very stable and chemically unreactive. Specifically:
- Helium (He) has 2 valence electrons, filling its only energy level (1s²).
- All other noble gases—Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn)—have 8 valence electrons, completing their outermost energy level (an octet)
This full valence shell configuration is the reason noble gases are the least reactive elements in the periodic table. Helium is an exception with only 2 valence electrons because its first shell can hold a maximum of 2 electrons, while the others follow the octet rule with 8 valence electrons in their outermost shell
