In Mendeleev's Periodic Table,gaps were left for elements to be discovered later. Which of the following elements found a place in the periodic table later?

The electronic configurations of $4$ elements $A, B, C,$ and $D$ are as follows:
$A: 2, 1$
$B: 2, 2$
$C: 2, 8, 2$
$D: 2$
$(a)$ Which of these elements belong to the same group? Name them.
$(b)$ Which of these elements belong to the same period? Name them.
$(c)$ Which of these elements is inert? Name it.

The elements $A$,$B$,$C$,$D$,and $E$ have atomic numbers $9$,$11$,$17$,$12$,and $13$ respectively. Which pair of elements belongs to the same group?

What would be the nature of oxides formed by the elements on the right-hand side of the periodic table?

Which among the following elements has the largest atomic radii?

Calcium is an element with atomic number $20$.
$(i)$ Will it be a metal or a non-metal?
$(ii)$ What will be its valency?
$(iii)$ What would be the formula of its chloride?
$(iv)$ Will it be larger or smaller than $K$ (Potassium)?