• # question_answer Two metals X and Y form covalent halides. Both halides can act as Lewis acids and a catalyst in Friedel Crafts reaction. Halide of X is polymer in the solid state and a dimer in the vapour state, which decomposes to monomer at 1200 K. However, halide of Y is a dimer in vapour state and becomes ionic in polar solvent. X and Y are respectively A) Be, Al B) Al, Be C) Be, Ca  D) Mg, Ca

Covalent halides are formed by Be and Al among the metals given. The halide of Al, ie, $A{{l}_{2}}C{{l}_{6}}$becomes ionic in polar solvent. Hence Y is Al. Halide of Be is a polymer in solid sta l te. Thus X is Be.