Find the atomic number of the element whose electronic configuration is $2, 8, 5$.

$Na, Mg, Al, Si, P, S, Cl$ and $Ar$ are the elements of the third period.
$(a)$ Which one of them is an inert gas? Why?
$(b)$ Which one of them has a valency of $4$? What kind of bond does it form: ionic or covalent?
$(c)$ Which of them are electropositive? Pick any two.

In the Modern Periodic Table,which are the metals among the first ten elements?

Which of the given elements $A$,$B$,$C$,$D$ and $E$ with atomic numbers $2$,$3$,$7$,$10$ and $30$ respectively belong to the same period?

$(a)$ State the main characteristics of elements on which the modern periodic table is based.
$(b)$ No fixed position can be assigned to hydrogen in the periodic table. Why?

The scientist who showed that the atomic number of an element is a more fundamental property than its atomic mass is: