Question

why do helium , neon and argon have zero valancy​

Answer 1

For,

• He: Because the total number of electrons in Helium is 2 and they are located in the K shell.
• Ar & Ne: The number of electrons in the outermost shell is 8, thus satisfy the octet rule.

More:

Conditions to become an inert:-

• The total number of electrons in the outermost shell must be 8.
• The total number of electrons in the shell should be same as (2n²) where, n = the number of layer of shell.