Which one is non-metal among the given elements, $\mathrm{C},\mathrm{Si},\mathrm{Ge},\mathrm{Sn},\mathrm{Pb}$.

The outermost subshell electronic configuration of an element is $3{\mathrm{d}}^5 4{\mathrm{s}}^2$.

Write the atomic number of the element.

Consider the following radioactive decays

I. ${}_{92}\mathrm{U}{\stackrel{-\mathrm{\alpha }}{⟶}}_{90}\mathrm{Th}$ and

II. ${}_{90}\mathrm{Th}\stackrel{-\mathrm{\alpha }}{⟶}{}_{88}\mathrm{Ra}$

In which case group of parent and daughter elements remains unchanged.

It is observed that characteristic X-ray spectra of elements show regularity. When frequency to the power 'n' i.e. ${\mathrm{v}}^{\mathrm{n}}$ of X-rays emitted is plotted against atomic number $\mathrm{Z}$, the following graph is obtained.

The value of 'n' is

Give scientific reasons: 

Elements belonging to the same group have the same valency.