
1888 summary of the usage of the affinity bond concept or "unit of affinity". [1]

1888 summary of the usage of the affinity bond concept or "unit of affinity". [1]

In chemistry, an affinity bond is an attachment between two atoms or two molecules.

Overview
In 1657, William Cullen introduced the crotchet notation "{" of the to represent a bond between to chemical species; as follows:

“The dart → between them expresses the elective attraction; when I put a dart with the tail to one substance and the point to another, I mean that the substance to which the tail is directed unites with the one to which the point is directed more strongly than it does with the one united to it in the crotchet {.”

which equates to the following diagrammatic logic, in respect to an AB affinity bonded chemical union put into contact with a new chemical species C, whereby B detaches from A, owing to stronger affinity forces, to form a new BC union:

In the 1880s, chemists would speak of “units of affinity”, “bond”, and “valence”, somewhat interchangeably, to the effect that, for example, carbon atoms have four valencies or four units of affinity. [1]

English chemist Matthew Muir summarized the situation in 1889 by saying that the symbol notation of H – C ≡ C – H would be used to represent the distribution of affinity, or the valencies, of the atoms of carbon and hydrogen in the molecule C2H2, which were becoming replaced, so to speak, by phrases to the effect that the carbons are linked by a triple bond to each other and by single bonds to the hydrogen atoms. Muir also explains how the translation of the concept of a "force of affinity", does not necessarily mean that force of affinity is divided into units or among the bonds, so to speak. [2]

The development of quantum chemistry in the early 20th century, in which bonding two atoms was defined, specifically by German physicist Walter Heitler in 1927, as the joining together of electron wavefunctions, with plus, minus, and exchange terms. [3]

References
1. Newberry, Spencer B. (1888). “The Significance of ‘Bonds’ in Structural Formulas”, Summarized Proceedings of the American Association for the Advancement of Science, 36: 140.
2. Muir, Matthew. (1889). A Treatise on the Principles of Chemistry (section: Consideration of possible meanings of expressions ‘bonds’ or ‘units of affinity’ as applied to atoms, pgs. 60-62, 132-37, esp. 133). Cambridge University Press.
3. Thims, Libb. (2007). Human Chemistry (Volume One) (pg. 250). Morrisville, NC: LuLu.