The order of increasing atomic size is:

$(a)$ Electropositive nature of the element$(s)$ increases down the group and decreases across the period.
$(b)$ Electronegativity of the element decreases down the group and increases across the period.
$(c)$ Atomic size increases down the group and decreases across a period (left to right).
$(d)$ Metallic character increases down the group and decreases across a period.
On the basis of the above trends of the Periodic Table,answer the following about the elements with atomic numbers $3$ to $9$:
$(a)$ Name the most electropositive element among them.
$(b)$ Name the most electronegative element.
$(c)$ Name the element with the smallest atomic size.
$(d)$ Name the element which is a metalloid.
$(e)$ Name the element which shows maximum valency.

$(a)$ 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.

On moving from left to right in a period in the periodic table,the size of the atom.

Name any two elements which were discovered later after Mendeleev's periodic table.

The element with atomic number $18$ belongs to which group?