In chemistry, affinity is the force of reaction or the degree to which two or more species are attracted. [1] The term "chemical affinity", according to English chemist and physicist Michael Faraday, is the force of chemical action between different bodies; that depends entirely upon the energy which particles of different kinds attract each other. [2] The term elective affinity was a common usage in the 18th century, whereas chemical affinity is the common modern usage. The first "affinity table", showing degrees of reactions between species, was the 1718 table made by French physician and chemist Étienne Geoffroy. [6]

In the history of chemistry, there are at least a dozen or more "laws of affinity", depending on which chemist is sourced. One of the first, was Plato's c. 390 BC affinity law that "like tends towards like".

See also
● Affinity of reaction

History
The name affinitas was first used in the sense of chemical relation by German philosopher Albertus Magnus near the year 1250. [2] In 1882, German physician and physicist Hermann von Helmholtz showed that free energy or maximum work, rather than the heat evolved, is the true measure of the affinity of a reactive chemical system, when a reaction is carried out irreversibly. [3] The influential 1923 textbook Thermodynamics and the Free Energy of Chemical Substances by American physical chemists Gilbert Lewis and Merle Randall led to the replacement of the term “affinity” by the term "free energy" in much of the English-speaking world. [4]

References
1. Thomson, Thomas. (1831). A System of Chemistry, vol. 1. (p.31: chemical affinity is described as an "unknown force"). 7th ed., 2 vols.
2. Faraday, M. (1861). On the Various Forces in Nature. New York: Prometheus Books.
3. Partington, J.R. (1937). A Short History of Chemistry, (pg. 322). New York: Dover (reprint).
4. Cahan, D. (1993). Hermann von Helmholtz – and the Foundations of Nineteenth-Century Science. Berkeley: University of California Press.
5. Leicester, H. (1971). The Historical Background of Chemistry. New York: Dover.
6. Thims, Libb. (2007). Human Chemistry (Volume Two), (preview), (ch. 10: “Goethe’s Affinities” and ch. 11: “Affinity and Free Energy”, pgs. 371-468) Morrisville, NC: LuLu.

Further reading
● Muir, Matthew M.P. (1907). A History of Chemical Theories and Laws (ch. XIV: Chemical Affinity, pgs. 379-430, esp. keyword: “Bergmann”, pgs. 384-94). Wiley.
● Kim, Mi Gyung. (2003). Affinity, That Elusive Dream – A Genealogy of the Chemical Revolution. Cambridge, Mass: The MIT Press.