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 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?

When Mendeleev started his work,the number of known elements were:

Mendeleev predicted the existence of certain elements not known at that time and named two of them as Eka-silicon and Eka-aluminium.
$(a)$ Classify these elements as metals,non-metals,or metalloids.
$(b)$ How many valence electrons are present in each one of them?

Where would you locate the element with electronic configuration $2, 8$ in the Modern Periodic Table?

Which of the following is the outermost shell for elements of period $2$?