Question

The correct order of atomic radii in group 13 element is _______
(A) B < Ga < Al < Tl < In
(B) B < AI < In < Ga < Tl
(C) B < Ga < Al < In < Tl
(D) B < Al < Ga < In < Tl

Answer 1

Hint: The size of atom is defined as distance between nucleus and outermost electron. It depends upon the number of shells and screening effect of orbitals. Generally, down the group the atomic size increases because new shells add at each step and effectively nuclear charge decreases.

Complete answer:
Atomic and ionic radii of group 13 elements are lower than those of alkaline earth metals of group 2 primarily due to greater nuclear charge of group 13 elements as compared to group 2 elements. On moving down the group the atomic radius of Ga is slightly lower than that of Al. This is due to the presence of d - electrons in Ga which do not shield the nucleus effectively. As a result, the electrons in Ga experience greater force of attraction by the nucleus than in Al and hence the atomic radius of Ga 135 pm is slightly less than that of Al 143 pm. Thus, the increasing order of atomic radii of group 13 elements is B < Ga < Al < In < Tl.

Hence the correct option is (C) B < Ga < Al < In < Tl.

Note: The boron family contains elements in group 13 of the periodic table and include the semi-metal boron (B) and the metals aluminium (Al), gallium (Ga), indium (In), and thallium (Tl). Aluminium, gallium, indium, and thallium have three electrons in their outermost shell (a full s orbital and one electron in the p orbital) with the valence electron configuration $ns^2np^1$. The elements of the boron family adopt oxidation states +3 or +1. The +3 oxidation states are favourable except for the heavier elements, such as Tl, which prefer the +1 oxidation state due to its stability; this is known as the inert pair effect.