anion. X-ray diffraction (XRD) A solution containing an aqua ion does not have the long-range order that would be present in a crystal containing the same ion, but there is short-range order. X-ray diffraction on solutions yields a radial distribution function from which the coordination number of the metal ion and metal-oxygen distance may be derived. With aqua ions of high charge some information is obtained about the second solvation shell. This technique requires the use of relatively concentrated solutions. X-rays are scatted by electrons, so scattering power increases with atomic number. This makes hydrogen atoms all but invisible to X-ray scattering. Large angle X-ray scattering has been used to characterize the second solvation shell with trivalent ions such as Cr3+ and Rh3+. The second hydration shell of Cr3+ was found to have molecules at an average distance of . This implies that every molecule in the first hydration shell is hydrogen bonded to two molecules in the second shell. Neutron diffraction Diffraction by neutrons also give a radial distribution function. In contrast to X-ray diffraction, neutrons are scatted by nuclei and there is no relationship with atomic number. Indeed, use can be made of the fact that different isotopes of the same element can have widely different scattering powers. In a classic experiment, measurements were made on four nickel chloride solutions using the combinations of 58Ni, 60Ni, 35Cl and 37Cl isotopes to yield a very detailed picture of cation and anion solvation. Data for a number of metal salts show some dependence on the salt concentration. †Figures in brackets are standard deviations on the last significant figure of the value.‡ angle between a M-OH2 bond and the plane of the water molecule. Most of these data refer to concentrated solutions in which there are very few water molecules that are not in the primary hydration spheres of the cation or anion, which may account for some of the variation of solvation number with concentration even if there is no contact ion pairing. The angle θ gives the angle of tilt of the water molecules relative to a plane in the aqua ion. This angle is affected by the hydrogen bonds formed between water molecules in the primary and secondary solvation shells. The measured solvation number is a time-averaged value for the solution as a whole. When a measured primary solvation number is fractional there are two or more species