Why do helium neon argon have a 0 valency?

Answer 1

Noble gases such as helium, neon, argon etc have completely filled valence orbitals, and hence have sable electronic configuration. They do not form bond with other elements. In other words, they are chemically inert and have zero valency.

Answer 2

Helium, neon, and argon have a full outer electron shell, making them stable and chemically non-reactive. They each have a valency of 0 because they do not need to gain, lose, or share electrons to attain a stable electron configuration.