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

$(a)$ Describe the main features of Bohr's model of an atom. Draw a neat and labelled diagram of energy levels.
$(b)$ Which of the following pairs are isotopes and which are isobars?
$(i)$ $_{26}^{58} A, _{28}^{58} B$
$(ii)$ $_{35}^{79} X, _{35}^{80} Y$
$(c)$ Elements $A$ and $B$ have atomic numbers $18$ and $16$ respectively. Which of these two would be more reactive and why?

$(a)$ Write the names of three isotopes of hydrogen.
$(b)$ State one use of an isotope of uranium.
$(c)$ Isotopes of an element have similar chemical properties. Why?

State two main postulates of Thomson's model of an atom.

The electronic configuration of an element $X$ is $2, 8, 2$.
$(a)$ Find the number of electrons present in the atom of element $X$.
$(b)$ Write the atomic number.
$(c)$ Is this element $X$ a metal or a non-metal?
$(d)$ Find out the valency of the element $X$.

Mention the postulates of Niels Bohr put forth to overcome the objections raised against Rutherford's atomic model.