The correct order of atomic radii in group 13 elements is

• B < Al < In < Ga < Tl

• B < Al < Ga < In < Tl

• B < Ga < Al < In < Tl

• B < Ga < Al < Tl < In

C.

B < Ga < Al < In < Tl

Atomic and ionic radii. Atomic and ionic radii of group 13 elements increase from top to bottom in the group. This is due to increase in the number of energy shells in each succeeding element. However, the atomic radius of gallium (Ga) is less than that of aluminium (Al).

It is due to the poor shielding of the valence electrons of Ga by the inner 3d-electrons. As a result, the effective nuclear charge of Ga is somewhat greater in magnitude than that of Al. Thus, the electrons in gallium experience the greater force of attraction by the nucleus than in aluminium. Hence the atomic size of Ga(135 pm) is slightly less than that of Al(143 pm).