Chemical Affinity

CHEMICAL AFFINITY, the "force" that causes substances to combine with one another, the "force" that holds heterogeneous elements together in a compound. The origin of the word "affinity" stems from the alchemical notion that for substances to combine they must have something in common, or be akin. According to modern views, the true measure of the affinity of substances entering into a reaction is the decrease in the free energy of the system, which is equal to the maximum work that the process can perform when proceeding under reversible conditions. The question of the very nature of chemical affinity forces, of the cause of the mutual attraction of atoms, is closely connected with the doctrine of atomic structure. Based on the Bohr atomic model, which assumes that the most stable electron shell is one consisting of eight electrons, Kossel and Lewis suggested that the atom of each element tends to form such a shell by easily giving up excess electrons or, conversely, by accepting missing ones. Thus, for example, a sodium atom has one electron in its outer orbit, while a chlorine atom has seven electrons; consequently, when a sodium atom comes into contact with a chlorine atom, the former easily gives up one electron and turns into a sodium ion with an 8-electron shell, similar to neon, but having a positive charge; the chlorine atom, having accepted an electron from sodium and thus completed its electron shell to 8, acquires a negative charge. The Na⁺ and Cl^- ions form an NaCl molecule due to mutual electrostatic attraction. Therefore, in such so-called heteropolar compounds, chemical affinity can be explained partially by electrostatic forces, although the very formation of ions from atoms occurs under the influence of a non-electrostatic factor. Less developed at present is the question of the nature of the homopolar bond, which plays a large role in organic compounds (H-H; -C-C-C-; C-H). It is assumed that in the case of a homopolar bond, two atoms share a pair of electrons describing orbits around both atomic nuclei. Recent research indicates that magnetic forces play an essential role in the homopolar bond, arising both from the motion of the electron in its orbit and as a result of the rotation of the electron around its axis. The concept of the intrinsic momentum of the electron (the so-called electron "spin") plays a major role in quantum mechanics, which seeks to explain the essence of chemical forces in homopolar bonds.