Which of the following pair of group 13 elements have similar atomic radius

Indium (In) and thallium (Tl) have similar atomic radii because of the phenomenon known as the inert pair effect. Indium and thallium both have valence electrons in the same outermost electron shell, but thallium has one more shell than indium.

The valence electrons in the outermost electron shell of In and Tl experience the same effective nuclear charge, which means that they are attracted to the nucleus with similar strength. However, the valence electrons in Tl are located farther away from the nucleus due to the additional electron shell, which can cause an increase in atomic radius.

However, this increase in atomic radius is offset by the inert pair effect, which is caused by the poor shielding of the valence electrons in the inner shells of heavy elements like Tl. As a result, the outermost electrons experience a stronger effective nuclear charge, making them less easily available for bonding and reducing the size of the atom. This effect is less pronounced in In due to its smaller atomic number and fewer electrons in the inner shells, so the atomic radius of In and Tl is relatively similar despite the difference in electron shells.