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A nonmetallic element of group $-13$, used in making bullet proof vests is extremely hard solid of black colour. It can exist in many allotropic forms and has unusually high melting point. Its trifluoride acts as Lewis acid towards ammonia. The element exhibits maximum covalence of four. Identify the element and write the reaction of its trifluoride with ammonia. Explain why does the trifluoride acts as a Lewis acid.
Solution
In group-$13$, boron is only non-metallic and extremely hard and also used for making bullet proof vests. Boron exists in many allotropic forms. It usually shows high melting point and does not have' $d$-orbital.
It can show maximum covalence of 4 by using $2 s$ and $2 p$ orbitals. In trivalent halides of boron, octet of boron is not completed hence acts as Lewis acid. It reacts with Lewis base and forms adduct.
