Valence

VALENCE (from Latin valere - to have value), or atomicity, is the number of hydrogen atoms or atoms or radicals equivalent to it, which a given atom or radical can attach. Valence is one of the foundations for the distribution of elements in the periodic system of D. I. Mendeleev: the maximum valence equals the number of the group in which the given element is located. Thus, elements of the zero group have zero valence, i.e., they do not combine with anything; elements of group I have one valence, i.e., they combine with one atom of H, Cl, and other monovalent elements; in group II, the valence is equal to two, and so on. Elements of groups with higher numbers have variable valence, i.e., the number determines only their maximum valence. Thus, S is hexavalent in the oxide SO₃, tetravalent in the oxide SO₂, and divalent in the compound H₂S. Phosphorus can be pentavalent (PCl₅) and trivalent (PCl₃), etc. Usually, valence has a polar character depending on which element, more positive or more negative, the given element combines with, and on the sign with which its ions pass into solution. Typical metals possess positive valences, and typical metalloids negative ones. These valences Abegg called normal. Besides them, each element possesses a valence of the opposite sign (according to Abegg, residual, or counter-valences). The sum of normal valences and counter-valences is equal to 8. Thus, for example, Cl possesses one strong negative valence (to H) and seven weaker counter-valences (to O₂). Compounds between molecules, leading to the formation of polymeric [n H₂O = (H₂O)_n] and complex (2KCl+PtCl₄ = K₂PtCl₆) compounds, are formed, according to Abegg, at the expense of residual valences. Werner gave a coherent theory of the formation of molecular compounds, which is currently one of the foundations of inorganic chemistry (see Coordination theory, Complex compounds). According to this theory, the indicated compounds are formed at the expense of auxiliary valences, which do not lead to the formation of ions. There exists a whole series of attempts to explain valence; the most successful explanation is given by the modern electronic theory of the structure of the atom (see), which considers it as a complex system consisting of a positive nucleus and a series of negative electrons. Elements of the zero group (noble gases) have in their outer shell the most stable grouping of 2 or 8 electrons. Atoms of other groups, having a less stable grouping of electrons, strive to approach the type of noble gases and either lose excess electrons (alkali metals - 1, alkaline earth metals - 2, etc.), passing into a mono- or di-charged positive ion, etc., or acquire the electrons they lack (e.g., halogens, possessing an outer shell of 7 electrons, strive to acquire an eighth, i.e., to become a monovalent negative ion - Cl). Upon their loss of the entire outer (incomplete) electron shell, their highest positive valence is realized. Thus, this scheme (Kossel) perfectly explains the valence and counter-valence of elements, as well as the formation of complex compounds. Supplemented by the concept of the distribution of outer electrons into subgroups (Main Smith, Stoner), it also explains variable valence.