• # question_answer 95)   A non-metallic 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 covalency of four. Identify the element and write the reasons of its trifluoride with ammonia. Explain why does the trifluoride act as a Lewis acid?

Non-metallic element of group 13 is boron. It is grey-black and very hard in nature. It has high melting point,$2300{}^\circ C$.It exists in two allotropic forms: (a) crystalline solid(b) amorphous powder. It forms trifluoride,$B{{F}_{3}}$.$B{{F}_{3}}$acts as a Lewis acid as it is an electron deficient compound. $B{{H}_{3}}+\,:N{{H}_{3}}\xrightarrow{{}}\underset{Adduct}{\mathop{{{H}_{3}}N}}\,\xrightarrow{{}}B{{F}_{3}}$ $N{{H}_{3}}$donates an electron pair which is accepted by boron to saturate its outer shell. Maximum covalency of boron is four as its valency shell contains only four orbitals.