Question

Why do the elements helium ,neon and argon have zero valency?

Answer 1

Because they are noble gases and their octets (duplet in case of helium) are already completed.Thus they show 0 valency.

Answer 2

Helium, Neon and Argon are noble elements in the periodic table, meaning their last shells or valence shells are completely filled with electrons hence are stable. ... they are very stable ,do not react with other element. certainly if no reaction takeplace ,SO NO transfer of electron. hence velency of He,Ar,Ne are zero.