Question

Of the following, which atom has the largest atomic radius?
A. $Cl$
B. $Br$
C. $Na$
D. $K$

Answer 1

Hint: Due to the addition of electrons to the same shell in all atoms, the atomic radius decreases on moving from left to right across the period in a periodic table. Sodium, potassium belongs to group one of the periodic table and are called alkali earth metals, while chlorine and bromine belong to group seventeen of the periodic table and are called halogens.

Complete answer:
Since sodium and chlorine are located in the same period i.e. period number three. Also, atomic size decreases on going from left to right along the period in the periodic table, therefore, sodium is larger in atomic size than chlorine because sodium is present at extreme left in the period and chlorine is present at the extreme right in the period. While sodium and potassium are present in the same group in the periodic table i.e. group number one and atomic radius increases down the group in the periodic table. Hence, potassium is larger in atomic radius than sodium.
Similarly, potassium and bromine are present in the same period and as the atomic radius decreases from left to right in a periodic table, therefore, potassium is larger in atomic radius than bromine because potassium is located at the extreme left in the period while bromine is located at the extreme right in the period. While chlorine and bromine are present in the same group i.e. group number 17 but atomic radius increases down the group in the periodic table hence chlorine is larger in atomic radius than bromine.
Therefore, the correct answer is option D, potassium is larger in atomic radius.

Note:
The horizontal rows in the periodic table are referred to as the period while the vertical columns are called the groups in the periodic table. The one-half of the distance between the nuclei of identical atoms that are bonded together is called the atomic radius which increases down the group and decreases across the period.