The hydration number of a compound is defined as the number of molecules of water bonded to a central ion, often a metal cation. The hydration number is related to the broader concept of solvation number, the number of solvent molecules bonded to a central atom. The hydration number varies with the atom or ion of interest.
In aqueous solution, solutes interact with water molecules to varying degrees. Metal cations form aquo complexes, wherein the oxygen of water bind to the cation. This first coordination sphere is encased in further solvation shells, whereby water bonds to the coordinated water via hydrogen bonding. For charged species, the orientation of water molecules around the solute dependent on its radius and charge, with cations attracting water's electronegative oxygen and anions attracting the hydrogens. Uncharged compounds such as methane can also be solvated by water and also have a hydration number. Although solvation shells can contain inner and outer shell solvent-solute interactions, the hydration number generally focuses on the inner shell solvent molecules that directly interact with the solute. Sodium ions are typically surrounded by 4 to 6 water molecules in their primary hydration shell. This arrangement reflects the ion's charge density and size, leading to strong ion-dipole interactions with water molecules. In contrast, chloride ions generally have a hydration number closer to 6 due to their larger ionic radius and more distributed charge, which allows them to stabilize a larger number of water molecules in their hydration shell.
