Question

Alcl3 has lower melting point than mgcl2.why?

Answer 1

According to fajjans rule greater the charge on cation more is the polarisation of anion ,more the polarisation of anion more is the covalent character.Hence AlCl3 is more covalent or we can say MgCl2 is more ionic . Since ionic compounds have higher melting point hence MgCl2 has higher melting point.

Answer 2

we know that,

covalent character is proportional to charge on cation and inversely proportional to size of cation

now,

size of Al < size of Mg

charge on Al (3+) > charge on Mg (2+)

therefore AlCl3 is more covalent than MgCl2

now higher the covalent character lower the melting point

therefore MgCl2 has higher melting point than AlCl3