In chemical physics and physical chemistry, chemical affinity is the electronic property by which dissimilar chemical species are capable of forming chemical compounds. Chemical affinity can also refer to the tendency of an atom or compound to combine by chemical reaction with atoms or compounds of unlike composition.

The concept of chemical affinity is an ancient idea, with origins in magic, predating science. Physical chemistry was among the first scientific disciplines to develop a "theory of affinity." The term "affinity" has been used since around 1600 to discuss structural relationships in various fields, including chemistry and philology. The concept has evolved throughout history, with notable chemists such as Albertus Magnus, Robert Boyle, and Antoine Lavoisier contributing to the development of the theory.

The first affinity table, which helped visualize chemical affinities, was published in 1718 by French chemist Étienne François Geoffroy. The table was a crucial tool in teaching chemistry to students and underwent several adaptations by leading chemists throughout the 18th century. The modern concept of chemical affinity refers to the electronic property that allows dissimilar chemical species to form compounds, as well as the tendency of atoms or compounds to combine through chemical reactions with unlike compositions.

Today, affinity is related to the phenomenon where certain atoms or molecules tend to bond or aggregate. The current IUPAC definition connects affinity with Gibbs free energy, where affinity is the negative partial derivative of Gibbs free energy concerning the extent of reaction at constant pressure and temperature. Belgian mathematician and physicist Théophile de Donder derived a relation between affinity and the Gibbs free energy of a chemical reaction, which was later expanded upon by Ilya Prigogine and Defay in their work, Chemical Thermodynamics.