Question

Assertion: Atomic radius of $Ga$ is larger than that of $Al$.
Reason: Atomic radius always increases down the group.
(A) Both Assertion and Reason are correct and Reason is the correct explanation of Assertion
(B) Both Assertion and Reason are correct and but Reason is not the correct explanation of Assertion
(C) Assertion is correct but Reason is incorrect
(D) Both Assertion and Reason are incorrect

Answer 1

Hint: You have to use the concept of screening and shielding, due to the poor shielding effect of d orbital size decreasing even after moving down the group. Energy level increases or in moving down the group atomic radius increases but it is some case atomic radius also decreases down the group.

Complete step by step solution:
From your chemistry lessons, you have learned about the screening and shielding effect. It is the phenomenon in which the nucleus reduces its force of attraction on the valence electrons due to the presence of electrons in the inner shell and they do not experience complete nuclear charge.
Shielding effects of d and f orbital are very poor because they do not experience complete nuclear charge due to the presence of inner electrons. If the distance of the electron from the nucleus increases shielding effect decreases.
As we know that moving down the group atomic radius increases but in some cases, it does not happen due to poor shielding of d and f orbitals.
Let us take the Assertion first: Atomic radius of Ga is larger than that of aluminium.
If you check according to the periodic table then gallium comes after aluminium, so the size should be larger but due to the poor shielding effect of d- orbital in gallium, its atomic radius is less than the aluminium. Because when we move from aluminium to gallium the electrons have already occupied the d- orbital(3d subshell) in gallium and thus the atomic radius of Ga becomes slightly lesser than aluminium.
As the no. of electrons increases down the group, energy level also increases and the distance of each energy level is further from the last one and therefore the atomic radius increases down the group.
So, the assertion is not correct because Ga has a lesser size than aluminium due to the poor shielding effect of d-orbital.
Reason: Atomic radius always increases down the group.
It is not compulsory that the atomic radius will always increase down the group as you have seen from the above example, Ga comes after aluminium but has a lower atomic radius.
So, the reason is also not correct.

Thus the correct option will be (D).

Note: On moving across the period shielding effect increases because atomic radius decreases and s orbital is more close to the nucleus so it will shield the other electrons from the effect of the nucleus and reduces the force of attraction on the outermost orbital. The order of shielding effect is $s$$>$$p$$>$$d$$>$$f$. The poor shielding effect of d orbital is also known as lanthanoid contraction.