Helium has 0 valency because the electron is 2 for helium and  the outer most shell is fully filled .... that is K shell contain 2 , so the valency is 0 for helium and it is known as noble gases or inert gases . its has extremely low reactive .
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Helium atomic no s 2 n itz electronic configuration s 2 which has oly 1 shell I.e., "K"....n itz has capacity 2 hold oly 2 electronz. in helium, diz shell s completely filled so it gained octate law. hence it s noble gas having 0 valency....

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