The alkaline earth metal nitrate that does not crystallize with water molecules is $\mathrm{Ba}{\left(\mathrm{NO}{)}_3\right)}_2$ (Barium nitrate).

This behavior is due to the size and hydration energy of the barium ion. As we move down the group in alkaline earth metals, the ionic size increases. Barium (${\mathrm{Ba}}^{2+}$) has the largest ionic size among the common alkaline earth metals. Its large size results in a lower charge density (charge/size ratio), which leads to a lower hydration energy. Hydration energy is the energy released when ions are surrounded by water molecules.

Because of its low hydration energy, barium nitrate does not form stable hydrated crystals. In contrast, the smaller ions like ${\mathrm{Mg}}^{2+}$, ${\mathrm{Ca}}^{2+}$, and ${\mathrm{Sr}}^{2+}$ have higher charge densities and higher hydration energies, so their nitrates do crystallize with water molecules (e.g., $\mathrm{Mg}{\left(\mathrm{NO}{)}_3\right)}_2·6H_{2}O$).

Related Topics

Alkaline Earth Metals and Properties: This group includes beryllium, magnesium, calcium, strontium, barium, and radium. Their properties, such as ionic size, hydration energy, and solubility, change predictably down the group.

Compounds of Alkaline Earth Metals: Nitrates, carbonates, sulfates, and other salts of these metals have distinct behaviors in terms of hydration, thermal stability, and solubility, which are influenced by the size and charge of the metal ion.

Hydration Energy: The energy released when an ion is hydrated. It decreases with increasing ionic size for ions of the same charge.

Formulae

Hydration Energy: It is not given by a simple formula but is inversely related to ionic radius for ions with the same charge.

Charge Density: $\frac{\mathrm{Charge}}{\mathrm{Ionic\; Radius}}$. Higher charge density leads to greater hydration energy.