Elements having the similar outer electronic configuration in their atoms are arranged in the same .

Give scientific reasons: 

Elements belonging to the same group have the same valency.  

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.

Select the correct set of statements regarding change in properties, as we move from left ot right in the second period of periodic table.

(i) Atomic size decreases

(ii) Valency remains the same

(iii) Electronegativity increases

(iV) Metallic character decreases

How many of the following statements are correct?

(a) '$\mathrm{He}$' is the second most abundant element in the universe.

(b) the symbol for the element with atomic number $110$ is $\mathrm{Ds}$.

(c) Osmium has the highest density among all elements.

(d) Francium is the most electropositive element in the periodic table.