Why do Helium, Neon and Argon have a zero valency?

Atomic number of helium is 2. Its electronic configuration is K-2. The maximum number of electrons K shell can accommodate is 2. It means that its duplet is complete. Helium atom does not have to gain or lose any electron since, its outermost shell is complete with 2 electrons. Hence, the valency of helium atom is zero.

Both Argon and Neon have 8 electrons in their outermost or their valence shells. It means their octet is complete. As their octets are complete, these elements do not have a tendency to gain or lose any electron. They do not tend to combine with other elements. This results in having a zero valency.