Question

Helium atom has 2 electrons in its valence shell but its valency is not 2. Explain.​

Answer 1

Helium atom has only one shell (K shell) which can have maximum 2 electrons. Thus, its shell is already complete called duplet. It can neither lose electrons nor gain electrons. Hence, its valency is 'zero'

Answer 2

because the 2 electrons are in the first shell and the first shell can only hold 2 atoms. that's the reason helium doesn't have any valency