$(i)$ The elements of the second period along with their atomic numbers in parentheses are given below: $B(5)$,$Be(4)$,$O(8)$,$N(7)$,$Li(3)$,$C(6)$,$F(9)$.
$(a)$ Arrange them in the same order as they appear in the periodic table.
$(b)$ Which element has the largest and smallest atom?
$(ii)$ Why does the atomic radius change as we move from left to right in a period?

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

Study the variation in the atomic radii of Group $I$ elements given below and arrange them in an increasing order.
$(i)$ Name the elements which have the smallest and the largest atoms.

Group $I$ elements

$Li: 52\, pm, Na: 61\, pm, K: 231\, pm, Rb: 244\, pm, Cs: 262\, pm$

$(ii)$ How does the atomic size vary as you go down a group?

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?

Study the following table in which positions of six elements $A, B, C, D, E$ and $F$ are shown as they are in the modern periodic table:

Group $\to$	$1$	$2$	$13$	$14$	$15$	$16$	$17$	$18$
Period $2$	$A$	-	-	-	$B$	-	-	$C$
Period $3$	-	-	$D$	$E$	-	-	-	$F$

On the basis of the above table,answer the following questions:
$(i)$ Name the element which forms only covalent compounds.
$(ii)$ Name the element which is a metal with valency three.
$(iii)$ Name the element which is a non-metal with valency three.
$(iv)$ Out of $D$ and $E$,which is bigger in size and why?
$(v)$ Write the common name for the family to which the elements $C$ and $F$ belong.

State the Modern Periodic Law.