Question

valency of noble gases is zero why

Answer 1

the noble gas are stable.but the other gas are not stable

Answer 2

heya!

noble gas are the one which have attained stability by gaining 8 or2 electrons in their outermost orbits...
there are 6 noble gases .. namely
helium
Krypton
xenon
Radon
argon
neon..

all these gases have valency zero because they have attached a stable gas configuration...
and they don't need more electrons..
why they have is sufficient..

hope this helps you