Question

Statement 1: Sodium has a largest atomic radius in the period after Argon.
Statement 2: Sodium is lighter than phosphorus.
(A) Both Statement 1 and Statement 2 are correct and Statement 2 is the correct explanation of Statement 1
(B) Both Statement 1 and Statement 2 are correct and Statement 2 is not the correct explanation of Statement 1
(C) Statement 1 is correct, but Statement 2 is not correct
(D) Statement 1 is not correct, but Statement 2 is correct

Answer 1

Hint: The statements are based on the periodicity trends of the periodic table. Thus, the trend will give the proper explanation for the variations with respect to sodium. Use the hint that atomic radius gradually increases as you go down the group.

Complete step by step solution:
Firstly, we shall be thorough with the periodicity trends of the modern periodic table to answer the given question. Thus,
Atomic radius along the period- Decreases from left to right.
Atomic radius along the group- Increases from top to bottom.
For statement I-
Sodium and Argon are the elements of the same period i.e. third period.
As we move across the period, electrons are added to the same shell at the same distance from the nucleus of the atom. Protons are also added along the period. The outer or valence electrons pull closer towards the nucleus as the positive charge in the nucleus increases resulting in the decreasing in the atomic radius across the period.
Thus, while moving in a period from left to right the atomic size decreases as the effective nuclear charge increases. But this is applicable only till halogen groups.
In the zeroth group the atom gets stabilised due to the complete octet in the outermost shell due to which effective nuclear charge becomes lesser than the effective nuclear charge of chlorine which makes argon atom bigger than chlorine.
But still the effective nuclear charge is greater than that of sodium due to which the size of the argon atom is smaller than that of the sodium atom.
For period three;
Metallic radii- Na, Mg, Al.
Covalent radii- Si, P, S, Cl.
Van der Waals radius- Ar as it does not form any strong bond.
It is fair to compare metallic and covalent radii because they both are both being measured in tightly bonded circumstances. It isn’t fair to compare these with a van der Waals radius though.
Then too, Sodium has a largest atomic radius in the period after Argon.
For statement II-
Also, sodium and phosphorus lie in the same period. Along a period, atomic radius decreases as the nuclear force of attraction increases. Sodium lies on the left-hand side of the period, while phosphorus lies in the right-hand side of the same period. Or also, the sodium’s atomic number is 11 whereas phosphorus’s atomic number is 15. Thus, the nuclear force of attraction will be more in case of phosphorus. Hence, sodium will have a larger radius than phosphorus. As we move along the period, atomic mass also increases.
Therefore, sodium is lighter than phosphorus.
For statement I and II-
Both the statements are correct but the statement II is not the explanation for statement I.

Hence, option (B) is correct.

Note: While answering these types of questions do note that we should be able to explain both the statements separately and then check if both can be related and also be considered as the explanation of each other.