1972) The bond lengths and angles in water molecules, derived from over 40 neutron-diffraction studies of crystalline hydrates, are analysed statistically. A 'quasi-normal' spread of the dimensions and con- sequent deviations from an average model, affecting both water molecules and their environment, is associated with strains due to local failures of Pauling's second rule. This interpretation is consistent with linear correlations between pairs of bond lengths and/or angles. A new, more general classification of water molecules in crystalline hydrates, based on cation coordination, is proposed. The water mol- ecules are arranged in five classes, according to the number of coordinated cations and to the position of the cations with respect to the lone-pair orbitals; each class may be further subdivided on the basis of the chemical nature of the cations. Introduction The water molecule plays an important role in the packing of crystalline hydrates, both because it partici- pates in hydrogen bonds linking anions and also be- cause, through its lone-pair orbitals, it is a satisfactory ligand for many cations. The approximately tetrahedral environment usually assumed by the water molecule has often been idealized and used for tentative estimates of the hydrogen-atom coordinates; constant molecular geometry and, some- times, linearity of hydrogen bonds and/or planarity of the water-acceptor group are assumed. In fact, both the conformation of the water molecule and the geom- etry of its environment do depend, to some extent, on the specific situations in different compounds. Neutron diffraction is the only technique that allows unambiguous location of hydrogen atoms of the water molecule in solid hydrates with estimated standard deviations (e.s.d.'s) comparable with those of the other atoms; while very accurate X-ray data may allow the location of hydrogen atoms, their e.s.d.'s are generally too high for quantitative discussion. Of the fifty or so crystal structures of hydrates (April 1972)studied in three dimensions by neutron diffraction, the results for 41 are reviewed here, with the 90 water molecules involved. Compounds with disorder in water molecules (cf. Ferraris, Jones & Yerkess, 1972b) or other atoms or atomic groups (cf. some alums) have been excluded. The appropriate planes (Fig. 1) and bond lengths and angles were recomputed and are summarized in Tables 1 and 2 according to a classification reported be- low. All distances and angles are uncorrected for thermal motion since, even when reported, such corrections are merely indicative. While Tables 1 and 2 deal with * Paper presented at the 6th Hungarian Conference on X-ray, Electron, and Neutron Diffraction; 28th May-lst June 1972, Si6fok, Hungary. Research supported by the C. N. R. all types of hydrogen bonds involving water molecules, the results to be discussed concern mainly O...O hydrogen bonds, since only for these does the number of cases studied by neutron diffraction allow statistical correlations. The following symbols are used (Fig. 1): W= oxygen atom of the water molecule; HI, H2 = hydrogen atoms of the water molecule; A 1,A2 = acceptors of hydrogen bonds; C1, C2, C3 = atoms contacting W; ~0 = H- W-H angle; tpl = A 1. • • W. • • A 2 angle; cq, ~2 = W-H- • • A angles; J~,Jz=angles between W...C1 and W-H; 7r = plane of the water molecule; zq = plane of 14, C2 and C3; ?1, Yz = angles between H- • • A and n; e~, ez, e3 = angles between W. • • C and n; e = C2. • • W. • • C3 angle; ~, = angle between ~z and nl; oA, o)2 =angles between the ~z-nl intersection straight line and W-H. ;~ ~C3