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

Why does atomic size decrease as we move from left to right along a period in the periodic table?

Predict the maximum number of valence electrons possible for atoms in the first period of the periodic table.

Atomic radii of the elements of the second period are given below :

Period $II$ elements	$B$	$Be$	$O$	$N$	$Li$	$C$
Atomic radius $(pm)$	$88$	$111$	$66$	$74$	$152$	$77$

$(i)$ Arrange them in decreasing order of their atomic radii.
$(ii)$ Are the elements now arranged in the pattern of a period in the Periodic Table?
$(iii)$ Which elements have the largest and the smallest atoms?
$(iv)$ How does the atomic radius change as you go from left to right in a period?

How many vertical columns are there in the modern periodic table and what are they called?

$(a)$ 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.