Animated Solution for Chemistry - s and p-Block Elements: The correct order of hydration enthalpies of alkali metal ions is

Hydration Enthalpies of Alkali Metals

When we look at the periodic table, the alkali metals in Group 1 present a fascinating study in trends. As we travel down the group from Lithium ($\text{Li}$) to Cesium ($\text{Cs}$), the addition of new principal electron shells causes the atomic and ionic radii to increase steadily.

This means that the bare gaseous ions follow a strict size order:

But what happens when these bare, gaseous ions are plunged into water? This is where the concept of Hydration Enthalpy comes into play.

Water is a highly polar molecule, meaning it has a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. When a positively charged alkali metal ion enters water, it acts like an electrostatic magnet, attracting the negative oxygen ends of the water molecules.

The energy released during this ion-dipole interaction is called the Hydration Enthalpy ($\Delta H_{\text{hyd}}$). The strength of this attraction depends entirely on the charge density of the ion.

Since all alkali metal ions carry an identical $+1$ charge, the charge density is inversely proportional to the ionic volume (or size).

Because the ${\text{Li}}^{+}$ ion is the smallest, its $+1$ charge is concentrated over a tiny volume, giving it an incredibly high charge density. It pulls water molecules towards itself with immense force, creating a massive, tightly bound hydration shell and releasing the maximum amount of energy.

Conversely, the ${\text{Cs}}^{+}$ ion is large and bulky. Its $+1$ charge is spread out, resulting in a low charge density. It can only weakly attract a few water molecules, releasing the least amount of energy.

Therefore, the order of hydration enthalpies is the exact reverse of their ionic sizes:

This leads us to a classic trap that examiners love to set. Because ${\text{Li}}^{+}$ attracts such a massive shell of water molecules, the hydrated Lithium ion (${\text{Li}}_{(aq)}^{+}$) is actually the largest and heaviest entity in the solution!

As a result, when an electric field is applied, the bulky hydrated Lithium ion moves the slowest. Always remember: the smallest bare ion becomes the largest hydrated ion, resulting in the lowest ionic mobility in an aqueous solution.