Why do helium, neon and argon have a zero valency?

Explanation

The electronic configuration of three elements are given:
`{:(,"Atomic No", K, L, M),("Helium (He)",2,2,-,-),("Neon (Ne)",10,2,8,-),("Argon (Ar)",18,2,8,8):}`
The first element helium (He) has only one shell and has maximum of two electrons in the only shell (K-shell) which it has.
Similary the next two elements neon (Ne) and argon (Ar) also have the maximum of eight electrons in their outermost shells L and M repectively
This means that the atoms of all the three elements donot have any range to either lose or gain electrons. Therefore they show zero valencies .These are also called noble gas or insert gas element as they do to take part in the chemical combination.