Hydrationnumber

Hydrationnumber, also referred to as the hydration number, is the average number of water molecules comprising the primary solvation shell around a solute in an aqueous solution. It describes how many water molecules are directly associated with the solute, typically through ion–water interactions, and are relatively tightly bound compared with bulk solvent. The concept is widely used for ions, though it can apply to other solutes that attract a hydration shell.

Determining the hydration number involves experimental and computational approaches. Techniques such as extended X-ray absorption fine structure (EXAFS), neutron and X-ray scattering, and various spectroscopy methods provide information on the first solvation sphere. Nuclear magnetic resonance (NMR) and infrared spectroscopy can offer complementary insights, while molecular dynamics simulations and quantum chemistry calculations yield time-averaged values and detailed solvent arrangements. Hydration numbers are often reported as averages due to rapid exchange of water molecules with the bulk.

The hydration number depends on several factors, including the ion’s charge density, size, and oxidation state, as well as temperature, pressure, and the presence of competing ligands or counterions. Water exchange with the bulk can be fast for some ions and slow for others, leading to variations in the observed number under different experimental conditions.

Typical values vary by ion: many small, highly charged cations exhibit hydration numbers around 6, while larger or lower-charged ions may have 4 to 8 in the first solvation shell. The concept is central to understanding solvation, ion transport, and reaction mechanisms in aqueous environments.