Liquid water is exceptional in having approximately as many hydrogen bonds as it has covalent bonds. In water''s hydrogen bonds, the hydrogen atom is covalently attached to the oxygen of a water molecule (492.2148 kJ mol^-1 ) but has (optimally) an additional attraction (about 23.3 kJ mol^-1a1 ; almost 5 x the average thermal collision fluctuation at 25°C)^a2 to a neighboring oxygen atom of another water molecule that is far greater than any included van der Waals interaction.ⁱ Hydrogen bonds within heavy water are stronger.^a3 Water''s hydrogen bonding holds water molecules up to about 15% closer than if than if water was a simple liquid with just van der Waals interactions. However, as hydrogen bonding is directional it restricts the number of neighboring water molecules to about four rather than the larger number found in simple liquids (for example, xenon atoms have twelve nearest neighbors in the liquid state. Formation of hydrogen bonds between water molecules gives rise to large, but mostly compensating, energetic changes in enthalpy (becoming more negative) and entropy (becoming less positive). Both changes are particularly large, based by per-mass or per-volume basis, due to the small size of the water molecule. This enthalpy-entropy compensation is almost complete, however, with the consequence that very small imposed enthalpic or entropic effects may exert a considerable influence on aqueous systems. It is possible that hydrogen bonds between para-H₂O, possessing no ground state spin, are stronger and last longer than hydrogen bonds between ortho-H₂O

The hydrogen bond in water is part (about 90%) electrostatic and part (about 10%) covalent and may be approximated by bonds made up of covalent HO-H····OH₂, ionic HO^δ--H^δ+····O^δ-H₂, and long-bonded covalent HO^-··H––O⁺H₂ parts with HO-H····OH₂ being very much more in evidence than HO^-··H––O⁺H₂, where there would be expected to be much extra non-bonded repulsion. Hydrogen bonding affects all the molecular orbitals even including the inner O1s (1a₁) orbital which is bound 318 kJ mol^-1 (3.3 eV) less strongly in a tetrahedrally hydrogen bonded bulk liquid phase compared to the gas phase [1227].

Clusters linked by extensive hydrogen bonding can be considered as being connected by extensive , but complementary, electron delocalization. As electrons are not held by individual molecules but are easily distributed amongst water clusters they can give rise to coherent regions  interacting with local electromagnetic radiation. The water protons are also not held by individual molecules but may switch partners in an ordered manner within distinct networks 

X-ray spectroscopic probing indicates that the electron transitions between molecular orbitals (changing with the local hydrogen bonding topology) with differing such contributions may shift on a time scale of less than a femtosecond. Contributing to the strength and dynamics of water''s hydrogen bonding are nuclear quantum effects (zero point vibrational energy) which bias the length of the O-H covalent bond longer than its ''equilibrium'' position length (as the shorter HO-H····OH₂ hydrogen bonds are stronger), so also increasing the average dipole moment [554]. On forming the hydrogen bond, the donor hydrogen atom stretches away from its oxygen atom and the acceptor lone-pair stretches away from its oxygen atom and towards the donor hydrogen atom [585], both oxygen atoms being pulled towards each other.

An important feature of the hydrogen bond is that it possesses direction; by convention this direction is that of the shorter O-H () covalent bond (the O-H hydrogen atom being donated to the O-atom acceptor atom on another H₂O molecule). In ¹H-NMR studies, the chemical shift of the proton involved in the hydrogen bond moves about 0.01 ppm K^-1upfield to lower frequency (plus about 5.5 ppm further upfield to vapor at 100°C); that is, becomes more shielded withreducing strength of hydrogen bonding as the temperature is raised; a similar effect may be seen in water''s¹⁷O NMR, moving about 0.05 ppm K^-1 upfield plus 36-38 ppm further upfield to vapor at 100°C.^b Increased extent of hydrogen bonding within clusters results in a similar effect; that is, higher NMR chemical shifts with greater cooperativity , shorter hydrogen bonded O-H····O distances , smaller atomic volume of the hydrogen atom, greater positive charge on the hydrogen atoms and greater negative charge on the oxygen atoms. The bond strength depends on its length and angle, with the strongest hydrogen bonding in water existing in the short linear proton-centered H₅O₂⁺ ion at about 120 kJ mol^-1. However, small deviations from linearity in the bond angle (up to 20°) possibly have a relatively minor effect . The dependency on bond length is very important and has been shown to exponentially decay with distance . Some researchers consider the hydrogen bond to be broken^c if the bond length is greater than 3.10 Å or the bond angle less than 146° ,^c2 although ab initio calculations indicate that most of the bonding energy still remains and more bent but shorter bonds may be relatively strong; for example, one of the hydrogen bonds in ice-four (143°). Similarly O····H-O interaction energies below 10 kJ mol^-1 have been taken as indicative of broken hydrogen bonds although they are almost 50% as strong as ''perfect'' hydrogen bonds and there is no reason to presuppose that it is solely the hydrogen bond that has been affected with no contributions from other interactions. Also, the strength of bonding must depend on the orientation and positions of the other bonded and non-bonded atoms and ''lone pair'' electrons .

There is a trade-off between the covalent and hydrogen bond strengths; the stronger is the H····O bond, the weaker the O-H covalent bond, and the shorter the O····O distance (see right). The weakening of the O-H covalent bond gives rise to a good indicator of hydrogen bonding energy; the fractional increase in its length determined by the increasing strength of the hydrogen bonding; for example, when the pressure is substantially increased (~ GPa) the remaining hydrogen bonds (H····O) are forced shorter causing the O-H covalent bonds to be elongated. Hydrogen bond strength can be affected by electromagnetic and magnetic effects.Dissociation is a rare event, occurring only twice a day that is, only once for every 10¹⁶ times the hydrogen bond breaks.

The anomalous properties of liquid water may be explained primarily on the basis of its hydrogen bonding