Animated Solution for Chemistry - s and p-Block Elements: Based on lattice energy and other considerations which one of the following alkali metal chlorides is expected to have the highest melting point ?

When we look at the melting points of ionic compounds, we are essentially measuring the strength of the invisible forces holding the crystal lattice together. Imagine a vast, three-dimensional grid of alternating positive and negative ions. The energy required to completely shatter this grid into isolated gaseous ions is known as the Lattice Energy ($U$).

Mathematically, lattice energy is governed by Coulomb's law. It is directly proportional to the product of the ionic charges and inversely proportional to the sum of their ionic radii:

For the alkali metal chlorides, the charges are always $+1$ and $-1$. Therefore, the lattice energy depends entirely on the size of the alkali metal cation. As we move down Group 1 from sodium ($N{a}^{+}$) to cesium ($C{s}^{+}$), the ionic radius increases. Consequently, the lattice energy decreases, and the melting point steadily drops. This explains why $NaCl$ melts at $801^{\circ }C$, $KCl$ at $770^{\circ }C$, and $CsCl$ at $645^{\circ }C$.

If we strictly follow the logic of lattice energy, lithium chloride ($LiCl$) should have the highest melting point because the lithium ion ($L{i}^{+}$) is the smallest alkali metal cation. However, chemistry is rarely a one-variable game.

This is where Fajans' Rules step onto the stage. According to Fajans, a very small cation with a high charge density possesses immense polarizing power. The tiny $L{i}^{+}$ ion acts like a powerful magnet, distorting the electron cloud of the much larger chloride ($C{l}^{-}$) ion. This distortion pulls the electron density into the space between the two nuclei, effectively creating a shared electron pair.

In other words, the bond in $LiCl$ acquires significant covalent character. Covalent bonds are highly directional, and compounds with high covalent character tend to form discrete molecules or weaker lattices compared to pure ionic solids. This covalent interference weakens the overall structural integrity of the $LiCl$ crystal, causing its melting point to plummet to around $605^{\circ }C$.

So, where does that leave us? Sodium ($N{a}^{+}$) is larger than lithium, meaning its polarizing power is significantly lower. The bond in $NaCl$ remains overwhelmingly ionic. At the same time, $N{a}^{+}$ is small enough to maintain a very high lattice energy compared to potassium, rubidium, and cesium.

Sodium chloride hits the perfect thermodynamic sweet spot: it maximizes lattice energy while minimizing covalent character. Therefore, among all the alkali metal chlorides, $NaCl$ boasts the highest melting point.