The main reasons are;
due to smaller size of F as compared  to Cl , six small F atoms can be easily accommodated  around si atom while cl cannot since f are present in smaller 2p orbital and cl are present in larger 3p orbital therefore interaction of f lone pair  electron with d orbitals of silicon are stronger in f as compared cl

diamond has three dimensional network structure involving strong c-c bond .these bonds are difficult to break and           hence melting point of diamond is very high 

because it does not release H+ions on its own it accepts OH- ion from the water molecule to complete its octet and in turn releasee H+ ion therefore it is a weak acid 

due to absence of d qrbitals b at the maximum can accommodate 8 electrons or 4 pairs of electron but not BF36 Iin which the covalency of b is  6