Helium has 2 electrons in its outermost orbit thus filling shell 1 and forming duplet configuration in the valence shell. Neon has 8 electrons in their valence orbit hence completing duplet configuration. In the same way, Argon and Neon have 8 electrons in its outermost shelling completing octet configuration. As these elements have a maximum electron in their valence shell thus reach stable electron configuration and they will not take part in any sort of chemical reactions.