Why does NaCl have a high melting point than KCl?

NaCl has a higher melting point than KCl because of the difference in the lattice energies of the two compounds. Lattice energy is the energy required to separate all the ions in a crystal lattice. The lattice energy of NaCl is higher than that of KCl because the Na+ and Cl- ions are more strongly attracted to each other than the K+ and Cl- ions. This is due to the fact that Na+ is a smaller ion than K+, so it has a higher charge density. The higher charge density of Na+ results in a stronger electrostatic attraction between the Na+ and Cl- ions.

In addition, the difference in the polarizing power of the Na+ and K+ ions also contributes to the difference in the melting points of NaCl and KCl. Polarizing power is the ability of an ion to distort the electron cloud of a neighboring ion. Na+ has a higher polarizing power than K+ because it is a smaller ion. The higher polarizing power of Na+ results in a greater distortion of the electron cloud of the Cl- ions in NaCl, which leads to a stronger electrostatic attraction between the Na+ and Cl- ions.

As a result of the higher lattice energy and the greater polarizing power of Na+, NaCl has a higher melting point than KCl.