Explain the following
(a) Gallium has higher ionisation enthalpy than aluminium
(b) Boron does not exist as `B^(3+)` ion
(c) Aluminium forms `[AlF_(6)]^(3-)` ion but boron does not form `[BF_(6)]^(3-)` ion.

Explanation

(a) In gallium, due to poor shielding of valence electrons by the intervening 3d electrons. The nuclear charge becomes effective, thus, atomic radius decreases and hence, the ionisation enthalpy of gallium is higher than that of aluminium.
(b) Due to small size of boron, the sum of its first three ionisation ethalpies is very high. This prevent it to form +3 ions and force it to form only covalent compound. That's why boron does not exist as `B^(3+)` ion
(c) Aluminium forms `[AlF_(6)]^(3-)` ion because of the presence of vacent d-orbitals so it can expant its coordination number from 4 to 6. In this complex. Al undergoes `sp^(3)d^(2)` hybridisation
On the other hand, boron does not form `[BF_(6)]^(3-)` ion, because of the unavailability of d-orbitals as it cannot expand its coordination number beyond four. Hence, It can form `[BF_(4)]^(-)` ion (`sp^(3)` hybridisation)