Oxyacids of the form HzXO4 (X = Cl, S, P, Si and Br, Se, As, Ge) were investigated with (1) the theories of atoms in molecules (AIM) and the electron localization function (ELF) at theB3LYP/6-311++G(3df,pd) level and with (2) G2 calculations. AIM and ELF analyses classify X-O bonds of 3rd-row oxyacids as a-bonded shared interactions with ELF disynaptic basin populations ranging from 1.39 to 2.17e. Those of 4th-row oxyacids have no ELF disynaptic basin populations, save X-OH bonds with smaller values ranging from 0.67 to only 1.07e. Both bonds are classified as mostly a-bonded closed-shelled interactions. The oxygen valence shell electron population in 3rd-row oxyacids range from 5.76 to 6.12e and from 7.86 to 7.96e in their 4th-row counterparts, and all OH disynaptic basin populations range from 1.70 to 1.79e. These populations have no correlation with the wide range of acid strengths of the oxyacids. The concept of underbonding, whereby the oxygen valence shell electron population is said to be proportional to acidity, therefore does not hold from this perspective. Acidity is better rationalized in terms of the electrostatic potential at H sites.
