$A$ helium atom has two electrons in its valence shell,but its valency is not two. Explain.
(N/A) The valence shell of a helium atom is the $K$-shell,which can hold a maximum of $2$ electrons. Since the $K$-shell is completely filled with $2$ electrons,the atom is stable and does not need to gain,lose,or share electrons to achieve a stable configuration. Therefore,its combining capacity or valency is $0$.
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