A group 13 element   ' X ' reacts with chlorine gas to produce a compound XCl 3 .   XCl 3 is electron deficient and easily reacts with NH 3   to   form   Cl 3 X ← NH 3 adduct; however, XCl 3 does not dimerize. X is:

Step-by-step solution

BCl 3 Boron react with chlorine gas to produce compound BCl 3 which is electron deficient. It does not dimerize because of small boron atom. It react with ammonia to form aduct. In it ammonia act as lewis base and BCl 3 act as lewis acid.