Using ab initio MO theory, a series of isodesmic reactions was studied in which nitrogen-, oxygen-, and fluorine-containing species acted as proton donors and accepters. Comparison of protonation and deprotonation energies suggests that approximately three-quarters of the enhanced acidity of acetic acid comes from electrostatic stabilization, while the remaining quarter results from pi resonance. Similar logic shows that only one-third of the enhanced acidity of the nitrogen analogue acetamidine is electrostatic in nature, and that the remaining two-thirds derives from resonance. The primary importance of electrostatics for oxygen and of pi resonance for nitrogen is further supported by the behavior of carbonic acid and guanidine. The contributions of hydroxy, amino, and fluorine substituents to the acidity and basicity of a series of alcohols and amines are well described by a simple electrostatic model with a single adjustable parameter. The model requires the electrostatic contributions of polar bonds to be additive, to be of equal magnitude but opposite sign for anions and cations, and to be strictly proportional to the electronegativity differences between the atoms comprising the bonds. Application of this model to a series of reactions lacking pi bonds results in a correlation coefficient of 0.99, and indicates that on average C-F bonds contribute 15 kcal/mol, C-O bonds 9 kcal/mol, and C-N bonds 4 kcal/mol to differential acidity and basicity. Further application of the model allows an estimation of pi resonance contributions to the acidity and basicity of acetic acid and a series of related compounds. These pi resonance contributions are found to be much greater for nitrogen than for oxygen, and significantly greater for acidity (anions) than for basicity (cations).