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

Explanation

Helium (He), neon (Ne) and argon (Ar) have completely filled outermost shell, i.e.,
`{:(" "K, " "K " "L),(""_(2)He to " "("duplet"), ""_(10)Ne to" "("octet")),(" "2, " "2 " "8):}`
`{:(" "K" "L" "M,),(""_(18)Ar to" "" "("octet"),),(" "2 " "8 " "8,):}`
Thus, they have stable electronic configuration. They neither lose electrons nor gain electrons. Hence, their valency is zero.