Question

why do elements helium,neon and argon have zero valency?

Answer 1

Because these gases are noble gases that's why they have their valency as zero......
hope it helps you ...

Answer 2

Hey Friend,
First of all, valency is the combining capacity of an element to form chemical bonds... or to become stable!

Now, to become stable an atom must complete its duplet or octet...
The elements such as helium, neon, argon etc.. have completed either duplet (helium) or octet (all others) and hence they are stable.
Since they are stable, they don't need any electrons to form chemical bonds in order to become stable!
So their valency is zero

Hope it helps!