What is the Debye–Hückel theory?
The Debye–Hückel theory, which I developed with Erich Hückel in 1923, describes how ions behave in electrolyte solutions. Let us consider a simple model: each ion is surrounded by an 'ionic atmosphere' of opposite charge, which screens its electrostatic field. This screening reduces the ion's effective charge and lowers its chemical potential. The theory yields a limiting law for the activity coefficient—a measure of non-ideal behavior—that depends on the square root of the ionic strength. For dilute solutions, this matches experiments remarkably well. We derived it by solving the Poisson–Boltzmann equation for a spherical ion, assuming the ions are point charges and the solvent is a continuous dielectric. The measurable consequence is that properties like conductivity and solubility deviate from ideal behavior in a predictable way. It remains a cornerstone of physical chemistry.
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