Question

why helium has zero valency while it has 2 valance electron?

Answer 1

Coz helium completes duplet. So now it can not able to give or take electrons from other elements so its valancy is 0
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Answer 2

it has 0 valency because

its outer most shell is completely filled

see valency and valance electrons are completely different

valance electrons are the electrons present in the outer most shell

valency is the ability to give or take electrons

for example): lets take oxygen

electronic configuration is 2 , 6

valence electrons = 6

valency = -2