The alum used for purifying water is

$A$ nonmetallic element of group $13$,used in making bulletproof vests,is an extremely hard solid of black colour. It can exist in many allotropic forms and has an unusually high melting point. Its trifluoride acts as a Lewis acid towards ammonia. The element exhibits a maximum covalence of four. Identify the element and write the reaction of its trifluoride with ammonia. Explain why the trifluoride acts as a Lewis acid.

Identify the reaction in which diborane is produced on an industrial scale.

Group $13$ elements show $+1$ and $+3$ oxidation states. The relative stability of the $+3$ oxidation state is given as:

Boron compounds act as Lewis acids due to .....

The formula of an oxyacid of phosphorus is $H_3PO_4$. It is a