with integral solvation numbers present in equilibrium with each other. This also applies to solvation numbers that are integral numbers, within experimental error. For example, the solvation number of 5.5 for a lithium chloride solution could be interpreted as being due to presence of two different aqua ions with equal concentrations. [Li(H2O)6]+ [Li(H2O)5]+ + H2O Another possibility is that there is interaction between a solvated cation and an anion, forming an ion pair. This is particularly relevant when measurements are made on concentrated salt solutions. For example, a solvation number of 3 for a lithium chloride solution could be interpreted as being due to the equilibrium [Li(H2O)4]+ + Cl− [Li(H2O)3Cl] + H2O lying wholly in favour of the ion pair. Vibrational spectra Infrared spectra and Raman spectra can be used to measure the M-O stretching frequency in metal aqua ions. Raman spectroscopy is particularly useful because the Raman spectrum of water is weak whereas the infrared spectrum of water is intense. Interpretation of the vibration frequencies is somewhat complicated by the presence, in octahedral and tetrahedral ions, of two vibrations, a symmetric one measured in the Raman spectrum and an anti-symmetric one, measured in the infrared spectrum. Although the relationship between vibration frequency and force constant is not simple, the general conclusion that can be taken from these data is that the strength of the M-O bond increases with increasing ionic charge and decreasing ionic size. The M-O stretching frequency of an aqua ion in solution may be compared with its counterpart in a crystal of known structure. If the frequencies are very similar it can be concluded that the coordination number of the metal ion is the same in solution as it is in a compound in the solid state. Dynamic methods Data such as conductivity, electrical mobility and diffusion relate to the movement of ions through a solution. When an ion moves through a solution it tends to take both first and second solvation shells with it. Hence solvation numbers measured from dynamic properties tend to be much higher that those obtained from static properties. {| class="wikitable" |+Hydration numbers measured by dynamic methods |- ! !! Li+!! Na+!!Cs+!!Mg2+!!Ca2+!!Ba2+!!Zn2+!!Cr3+||Al3+ |- | Ion transport number||13-22||7-13||4||12-14||8-12||3-5||10-13|| || |- | Ion mobility||3-21||2-10|| ||10-13||7-11||5-9||10-13|| || |- | Diffusion||5||3||1||9||9||8||11||17||13 |} Solvation numbers and structures Hydrogen Hydrogen is not a metal, but like them it tends to lose its valence electron in chemical reactions, forming