Factors Affecting Melting Point of Alkali Metal Chlorides

Lattice Energy:
The lattice energy is a measure of the strength of the ionic bonds in a crystal lattice. It is the energy released when one mole of an ionic compound is formed from its constituent ions in the gas phase. The higher the lattice energy, the stronger the ionic bonds, and the higher the melting point of the compound.

Size of the Cation:
The size of the cation plays an important role in determining the melting point of alkali metal chlorides. As we move down the group, the size of the cation increases due to the addition of extra electron shells. This increase in size leads to a decrease in the strength of the ionic bond and a decrease in the lattice energy.

Polarizing Power:
The polarizing power of the cation is the ability of the cation to distort the electron cloud of the anion. The greater the polarizing power, the stronger the ionic bond formed and the higher the lattice energy. As we move down the group, the polarizing power of the cation decreases due to the increase in size and the shielding effect of the inner electron shells.

Explanation of the Correct Answer (Option C: NaCl)

In the given options, the correct answer is NaCl. Here's why:

- Lattice Energy: NaCl has a higher lattice energy compared to the other alkali metal chlorides (RbCl, KCl, and LiCl) due to the smaller size of the Na+ cation compared to the other cations. The smaller size of Na+ allows for stronger electrostatic attractions between the cation and the anion, resulting in a higher lattice energy.

- Size of the Cation: As we move down the group from Li to Rb, the size of the cation increases. Therefore, RbCl has the largest cation size among the given options. The larger the cation, the weaker the ionic bond formed, leading to a lower lattice energy and a lower melting point.

- Polarizing Power: As mentioned earlier, the polarizing power decreases as we move down the group. Therefore, Na+ has a higher polarizing power compared to Rb+ and K+. This higher polarizing power strengthens the ionic bond in NaCl, resulting in a higher lattice energy and a higher melting point compared to RbCl and KCl.

In summary, NaCl is expected to have the highest melting point among the given options due to its higher lattice energy resulting from a smaller cation size and higher polarizing power.

As we go down in the group, ionic character increases hence, melting point of halides should increase but NaCl has the highest melting point (800℃) due to its high lattice energy.