Aluminium chloride in acidified aqueous solution forms an ion having geometry

The bond dissociation energy of $B- F$ in $BF_3$ is $646 \,kJ\, mol^{-1}$ whereas that of $C – F$ in $CF_4$ is $515\, kJ\, mol^{-1}.$ The correct reason for higher $B- F$ bond dissociation energy as compared to that of $C-F$ is

The Lewis acid character of boron tri halides follows the order:

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.

${H_3}B{O_3}\,\xrightarrow{{{T_1}}}X\,\xrightarrow{{{T_2}}}Y\,\xrightarrow{{\operatorname{Re} d\,\,\,hot}}{B_2}{O_3}$

if $T_1 < T_2$ then $X$ and $Y$ respectively are