Question

wy elements of group 18 are called zero valent

Answer 1

That's because the elements under group 18 are all noble or inert gases.. they are already complete and will not go under any reaction. They have octave configuration... So they are called as zero valent as their valency is 0. There's no need to gain or lose any electrons ..

Thank you! Hope it helps....

Answer 2

because in their outermost orbit , the octet is complete. octet means in last orbit a element need 8 electrons to stable.
so see the division of 18 electrons as 2,8,8
so their valency is 8 and valence or valent is zero