Why do helium, neon and argon have a zero valency?

1.  ${}_{10}{}^{20}\mathrm{Ne}$,  ${}_{10}{}^{21}\mathrm{Ne}$ and  ${}_{10}{}^{22}\mathrm{Ne}$ are the isotopes of neon.
(i) What do the subscripts and superscripts represent?
(ii) What is the cause of change in superscripts?
(iii) Give the geometric diagram of  ${}_{10}{}^{22}\mathrm{Ne}$.

What information do you get from the figure given about the atomic number, mass number and valency of atoms X, Y, and Z?. Give your answer in a tabular form.

The average atomic mass of a sample of an element X is 23.04 m. What are the percentage of isotopes ${}_{11}{}^{23}\mathrm{X} \mathrm{and} {}_{11}{}^{24}\mathrm{X}$ in the sample?