Question

Helium atom has 2 electrons in its valence shell but it's valency is not 2 explain

Answer 1

helium is a Nobel gas

Answer 2

It's right that Helium has 2 valence shells , so a helium atom has only 1 shell which is completely filled , means a helium atom is stable and thus no need to react or combine with any other element . Due to its stability Helium atom exists independently.