(A) : In IIIA group ionization enthalpy do not decrease smoothly down the group.
(R) : In the heavier elements of IIIA group the ‘d’ and ‘f ’ electrons have poor shielding effect.

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Both (A) and (R) are true and (R) is the correct explanation of (A)

(A) is true but (R) is false

Both (A) and (R) are false

Both (A) and (R) are true and (R) is not the correct explanation of (A)

answer is A.

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Detailed Solution

I.E. Order: B >Tl >Ga > Al > In

Atomic radius order: Tl $\approx$ In >Al > Ga > B

B < Ga < Al < In ~ Tl

Ionization energy order of IIIA group elements is, B >Tl >Ga > Al > In

On moving down the group, the ionization energy decreases, this is true for B and Al. The ionization energy of Ga is unexpectedly higher than that of Al.

This is due to poor shielding effect by completely filled d-orbitals in the inner shell of Ga, hence the Ga valency electrons are strongly held by nucleus therefore removal of electron from Ga is highly difficult compared to Al.

A similar increase is observed from In to Tl due to presence of 14 f-orbital electrons in the inner shell of Tl which have poor shielding effect.

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