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?

Two elements $P$ and $Q$ belong to the same period of the modern periodic table and are in Group $1$ and Group $2$ respectively. Compare their following characteristics in tabular form:
$(a)$ The number of electrons in their atoms
$(b)$ The sizes of their atoms
$(c)$ Their metallic characters
$(d)$ Their tendencies to lose electrons
$(e)$ The formula of their oxides
$(f)$ The formula of their chlorides

The electronic configuration of an element $X$ is $2, 8, 8, 2$. To which $(a)$ period and $(b)$ group of the modern periodic table does $X$ belong? State its valency. Justify your answer in each case.

$(a)$ Why do we classify elements?
$(b)$ What were the two criteria used by Mendeleev in creating his Periodic Table?
$(c)$ Why did Mendeleev leave some gaps in his Periodic Table?
$(d)$ In Mendeleev's Periodic Table,why was there no mention of noble gases like Helium,Neon,and Argon?
$(e)$ Would you place the two isotopes of chlorine,$Cl-35$ and $Cl-37$,in different slots because of their different atomic masses or in the same slot because their chemical properties are the same? Justify your answer.

Why does the basic character decrease along a period? Give reasons.

Which among the following elements has the largest atomic radii?