Why do the elements helium, neon …

Answer:
Helium has two electrons in its energy shell, while argon and neon have 8 electrons in their valence shells. As these have maximum number of electrons in their valence shells, they do not have any tendency to combine with other elements. Hence, they have a valency equal to zero.

Valency is defined as the measure of the combining power with other atoms when it forms a molecule of compound. ... Noble gases have valency 0 because these elements cannot loose or gain electrons. The electrons are fully filled in the sub-shells of these elements. hence, the valency of these elements is 0. Helium has two electrons in its only energy shell. ... As the elements have the maximum number of electrons in their valence shells (thereby achieving stable duplet and octet configurations), they do not have any tendency to combine with other elements. Hence, Helium, Neon, and Argon have zero valency.