An element $A$ has an atomic number of $17$. To which group and period does it belong?

$(a)$ 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.

Four elements $P, Q, R$ and $S$ have atomic numbers $12, 13, 14$ and $15$ respectively.
Answer the following questions giving reasons:
$(i)$ What is the valency of $Q$?
$(ii)$ Classify these elements as metals and non-metals.
$(iii)$ Which of these elements will form the most basic oxide?

If an element $X$ is placed in group $14$,what will be the formula and the nature of bonding of its chloride?

Mendeleev predicted the existence of certain elements not known at that time and named two of them as Eka-silicon and Eka-aluminium.
$(a)$ Name the elements which have taken the place of these elements.
$(b)$ Mention the group and the period of these elements in the Modern Periodic Table.

Arrange the following elements in the increasing order of their metallic character:
$Mg, Ca, K, Ge, Ga$