Question

Arrange the elements in increasing order of atomic radius${\text{Na,Rb,K,Mg}}$.

A.${\text{Na < K < Mg < Rb}}$
B.${\text{K < Na < Mg < Rb}}$
C.${\text{Mg < Na < K < Rb}}$
D.${\text{Rb < K < Mg < Na}}$

Answer 1

Atomic radius is the distance between the nucleus and outermost shell of electrons. In the periodic table, the atomic radius decreases moving from left to right, across the period and increases moving down the group, from top to bottom.

Complete Step by step answer: Determine the group and period number of all given elements.

Element	Group Number	Period Number
${\text{Na}}$	IA	3
${\text{K}}$	IA	4
${\text{Rb}}$	IA	5
${\text{Mg}}$	IIA	3

Out of the elements given to us, an element ${\text{Mg}}$ is present in IIA group of the periodic table while other elements ${\text{Na,Rb}}$ and ${\text{K}}$ are present in IA group of the periodic table.
As per the trend in the periodic table atomic radius increases down the group so the increasing order of atomic radius of IA group element given to us is as follows :
${\text{Na < K < Rb}}$
As per the trend in the periodic table atomic radius decreases moving from left to right across the period so out of ${\text{Na}}$ and ${\text{Mg}}$ that are present in the same period ${\text{Mg}}$ has the lowest atomic radius than ${\text{Na}}$ as it is the present right side (IIA group) to element ${\text{Na}}$( IA group).
Thus, the increasing order of atomic radius of a given element is ${\text{Mg < Na < K < Rb}}$

Hence the correct option is (C) ${\text{Mg < Na < K < Rb}}$.

Note: In moving down a group atomic radius increases successively due to the increase in main energy level (K, L, M, N...) and nuclear charge. In passing along the period from left to right the atomic radius decreases due to an increase in nuclear charge without the addition of a new shell.