A physical model of the hydrogen bond, A-H⋯B, has been deduced from ab initio molecular orbital wave functions of 36 dimers made from the monomers, NH3, OH2, FH, PH3, SH2, and ClH. Three monomer quantities are defined which characterize the model: μA-H, the A-H bond dipole; ΔI, the difference between first ionization potentials of the electron donor and the noble gas atom in its row; and l, the length of the hydrogen bonding lone pair. Dimerization energy, charge transfer, internuclear separation, directionality, stretching force constants (KAB and KAH), the dimer dipole moment, and ir intensity enhancement can be understood in terms of these quantities. The dimerization energy formula, ED = KμA-HΔI/R, where K is an energy scale factor and R, the internuclear separation between A and B, systematizes existing experimental and computational data. The tendency for strong bonding electron donors to be weak bonding proton donors and vice versa is the result of an intrinsic reciprocal relationship between μA-H and ΔI. Charge transfer is proportional to μA-H for specified B, and is ordered according to l for a given A. Internuclear separation is inversely proportional to μA-H for specified B, and has close to the same dependence on A-H for second- and third-row electron donors. The almost constant separation difference of 0.8 Å between second- and third-row electron donors results from the difference in average l between the rows. The rule of constant R for all B in a row (with given A) is found to arise from the constancy of l times I. Stretching force constants for the heavy atoms follow Badger's rule, KAB(R - dAB)3 = 1.86, with dAB dependent only on the column of the periodic table. dAB is 1.00, 0.80, and 0.55 Å for groups 5, 6, and 7, respectively. Lowering of the A-H stretching force constant, KAH, relative to the monomer, is proportional to μA-H for fixed B, variable A, and proportional to ΔI (or l) for fixed A, variable B. The model also provides qualitative explanations and some quantitative results for the properties of other hydrogen bonds: the strong hydrogen bonds found in crystal ions, the weak hydrogen bonds to π electrons in organic molecules, the multiply bonded electron donors of proteins, a variety of substituents at A and B, and the cooperativity found in trimers and higher polymers. Quantitative predictions of ED and R can be made for dimers formed with fourth-row hydride monomers.