Discuss the characteristics of group 13 elements in terms of:
(i) Atomic and ionic radii
(ii) ionisation enthalpy
(iii) Density, melting point and boiling points.

(i) 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).
(ii) It should be noted that there is a very small increase in the atomic radii of gallium and indium. This is due to inert pair effect i.e. ineffective shielding of the valence shell by the intervening d and f-electrons.

(iii) Ionisation enthalpy. As we move down the group from B to Al, the ionisation enthalpy decreases due to the increase in atomic size and screening effect. However, gallium has higher ionisation enthalpy than aluminium.

(iii) Density, melting point and boiling point. The density of these elements increases as we move from top to bottom in the group. This is due to increase in the atomic mass of the element which outweighs the effect of an increase in atomic size.
The melting points in this group decrease considerably on moving down the group up to gallium and then increase in case of indium and thallium. Boiling points decrease regularly from boron to thallium.