Answers

2016-04-01T20:23:37+05:30
Valency of an element depends on the number of valence electrons.

since its outermost shell is full so it does not need to gain or loose any electrons to be stable. and therefore it is an inert gas ( inert gases are those gases which have 0 as its valency ) .

i hope it helps...............       
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2016-04-01T20:24:55+05:30
Because the no. of valence electron in the k shell is 2 and that is why 2 valence electrons in the heliums outermost shell make it noble gas and it is unreactive in nature that is why its valency is 0
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