Which of the given elements $A, B, C, D$ and $E$ with atomic numbers $2, 3, 7, 10$ and $30$ respectively belong to the same period?

Justify the following statements:
$(a)$ Cations are smaller in size than the corresponding neutral atom.
$(b)$ The size of the atom increases as we move down a group.
$(c)$ Atomic size decreases as we move across a period.

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?

$(i)$ 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?

"Hydrogen occupies a unique position in the Modern Periodic Table". Justify the statement.

Given below are four elements with their atomic numbers:

Element	Atomic Number
$A$	$16$
$B$	$11$
$C$	$3$
$D$	$14$

$(a)$ Identify the elements which belong to the same group of the Modern Periodic Table.
$(b)$ Arrange the given elements in decreasing order of atomic size.
$(c)$ Write the formula of the oxide of $B$.
$(d)$ Which of the above elements is a metalloid?