A nonmetallic element of group 13, used in making bulletproof vests is extremely hard solid of black colour. It can exist in many allotropic forms and has an 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 reaction of its trifluoride with ammonia. Explain why does the trifluoride act as a Lewis acid.
Element is Boron which is very hard and black in colour. It's melting point is very high.
BF3 + NH3 → BF3ß NH3
Monomeric trihalides have deficient electrons so they act as strong lewis acid. To complete the octet of boron, trifluoride of boron easily react with Lewis bases such as NH3. It is due to the absence of d electrons in boron, it cannot exceed the covalence above 4.