Why melting point of LiCl is lower than NaCl?

Answer 1

Using polarizing theory:

Li+ has a smaller atomic radius than Na+. However they both have the same charge, +1. The Cl- is identical in both compounds, and has a large electron cloud surrounding it. Using the equation for static force:

F=kq1q2/r2 where k is Coulomb's constant, q1 is the charge of the Li+ or Na+ cation, q2 is the charge of an electron in the chlorine electron cloud, and r is the distance between the chlorine electron and the Li+ or Na+ nucleus.

we see that the force increases with a smaller radius r. Since the Li+ ion is smaller, the chlorine electrons are closer to the nucleus, and thus these electrons are drawn even closer to the Li+ nucleus.

All together, the electrons from the chlorine in LiCl are pulled around the Li+ ion to a much further extent than they are pulled around the Na+ in NaCl. Therefore the bond in LiCl has a much more covalent character than NaCl. Bond polarity is also reduced, since the electrons are not spending all of their time around the Cl-.

Since the LiCl molecules are less polar than NaCl, there are less electrostatic forces holding them together in a solid crystal lattice. Therefore the energy required to loosen the molecules from a crystal to a liquid is less for LiCl than NaCl.

Finally we come to the conclusion that therefore, the melting temperature of LiCl must be lower than NaCl

Answer 2

The melting point of a compound is influenced by the strength of the bonding between its constituent particles. Since LiCl has smaller ions compared to NaCl, the electrostatic forces of attraction between Li+ and Cl- ions are weaker than those between Na+ and Cl- ions. This leads to a lower melting point for LiCl compared to NaCl.