Explain the following :
(a) Boron has high melting and boiling points.
(b) The `p pi - p pi` back bonding occurs in the halides of boron and not in those of aluminimum.
( c) Boron and aluminium halides behave as Lewis acids.
(d) Aluminium forms `[AlF_(6)]^(3-)` ion, but boron does not form `[BF_(6)]^(3-)` ion.

Explanation

(i) The `Delta_(1)H_(1)` value of `Ga(578kJ"mol"^(-1))` is slightly higher than that of `Al(577kJ "mol"^(-1))` wheras, it is expected to be less. Acutally, gallium atom has ten 3d electrons which do not exert as much screening efect as is done by a and p electrons in case of alumimum. Therefore there is an unexpected inc rease in the magnitude of effective nuclear charge as we move from aluminum to gallium and as a result, the `Delta_(1)H_(1)` of gallium is little more than that of aluminium, inspita=e of the fact that atomic size of aluminium is less.
(ii) The size of boron atom is very small, in order to form `B^(3+)` ion, the element needs a very high ionisation enthalpy `(Delta_(1)H_(1)+Delta_(1)H_(2)+Delta_(1)H_(3))` This is not availab le easily. therefore, It does not form `B^(3+)` ion or the compounds of the element are not ionic, therese are generally covalent in nature.
(iii) For answer, Consult concept based Queston (Q.1)