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Valency of helium is zero although it contains 2 electrons in it's outer most orbit. this is due to it's stability which prevents it to lose or gain of it's electrons.
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According to duplet rule if an atom have 2 electrons in the outermost shell then it said to be stable and helium have 2 electrons in its outermost shell so it is stable and due to this it said to be having ZERO valency.
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