a cation H+. In aqueous solution, this immediately attaches itself to a water molecule, forming a species generally symbolised as H3O+ (sometimes loosely written H+). Such hydration forms cations that can in essence be considered as [H(OH2)n]+. The solvation of H+ in water is not fully characterised and many different structures have been suggested. Two well-known structures are the Zundel cation and the Eigen cation. The Eigen solvation structure has the hydronium ion at the center of an complex in which the hydronium is strongly hydrogen-bonded to three neighbouring water molecules. In the Zundel complex the proton is shared equally by two water molecules in a symmetric hydrogen bond. Alkali metals The hydrated lithium cation in water is probably tetrahedral and four-coordinated. There are most probably six water molecules in the primary solvation sphere of the octahedral sodium ion. Potassium is seven-coordinate, and rubidium and caesium are probably eight-coordinate square antiprismatic. No data is available for francium. Alkaline earth metals Other values include Zn2+ -2044.3, Cd2+ -1805.8 and Ag+ -475.3 kJ mol−1. There is an excellent linear correlation between hydration enthalpy and the ratio of charge squared, z2, to M-O distance, reff. Values for transition metals are affected by crystal field stabilization. The general trend is shown by the magenta line which passes through Ca2+, Mn2+ and Zn2+, for which there is no stabilization in an octahedral crystal field. Hydration energy increases as size decreases. Crystal field splitting confers extra stability on the aqua ion. The maximum crystal field stabilization energy occurs at Ni2+. The agreement of the hydration enthalpies with predictions provided one basis for the general acceptance of crystal field theory. The hydration enthalpies of the trivalent lanthanide ions show an increasingly negative values at atomic number increases, in line with the decrease in ionic radius known as the lanthanide contraction. Single ion hydration entropy can be derived. Values are shown in the following table. The more negative the value, the more there is ordering in forming the aqua ion. It is notable that the heavy alkali metals have rather small entropy values which suggests that both the first and second solvation shells are somewhat indistinct. {| class="wikitable" style=text-align:center |+Single ion standard hydration entropy at 25 °C /J deg−1 mol−1 | Li+-118.8|| |- |Na+-87.4||Mg2+-267.8|| || ||Al3+-464.4 |- |K+-51.9||Ca2+-209.2|| || ...||Ga3+-510.4 |- |Rb+-40.2||Sr2+-205.0|| ||... ||In3+-426.8 |- |Cs+-36.8||Ba2+-159.0 ||La3+-368.2||... || |} Hydrolysis of aqua ions There are two ways of