Name any two noble gases.

What are isobars? Give one example.

With the help of a labelled diagram,describe in brief the Rutherford's alpha particle scattering experiment. Write any three important conclusions drawn from the experiment.

Write the atomic number of an element which has mass number $23$ and number of neutrons $12$ in the nucleus.

$(a)$ Calculate the number of electrons,protons,and neutrons in an atom of an element with atomic number $20$ and mass number $40$. Write the electronic configuration and draw the structure of the atom.
$(b)$ An atom has complete $K$ and $L$ shells. Is this an atom of a metal,non-metal,or noble gas? Justify.

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