Valence (chemistry)

[4] The concept of valence was developed in the second half of the 19th century and helped successfully explain the molecular structure of inorganic and organic compounds.

In 1789, William Higgins published views on what he called combinations of "ultimate" particles, which foreshadowed the concept of valency bonds.

The exact inception, however, of the theory of chemical valencies can be traced to an 1852 paper by Edward Frankland, in which he combined the older radical theory with thoughts on chemical affinity to show that certain elements have the tendency to combine with other elements to form compounds containing 3, i.e., in the 3-atom groups (e.g., NO3, NH3, NI3, etc.)

According to him, this is the manner in which their affinities are best satisfied, and by following these examples and postulates, he declares how obvious it is that[6] A tendency or law prevails (here), and that, no matter what the characters of the uniting atoms may be, the combining power of the attracting element, if I may be allowed the term, is always satisfied by the same number of these atoms.This "combining power" was afterwards called quantivalence or valency (and valence by American chemists).

Lothar Meyer in his 1864 book, Die modernen Theorien der Chemie, contained an early version of the periodic table containing 28 elements, for the first time classified elements into six families by their valence.

An alternative definition of valence, developed in the 1920's and having modern proponents, differs in cases where an atom's formal charge is not zero.

In 1916, Gilbert N. Lewis explained valence and chemical bonding in terms of a tendency of (main-group) atoms to achieve a stable octet of 8 valence-shell electrons.

Pauling also considered hypervalent molecules, in which main-group elements have apparent valences greater than the maximal of 4 allowed by the octet rule.

Many elements have a common valence related to their position in the periodic table, and nowadays this is rationalised by the octet rule.

Polyvalence or multivalence refers to species that are not restricted to a specific number of valence bonds.

Unlike Cs and Ca, Fe can also exist in other charge states, notably 2+ and 4+, and is thus known as a multivalent (polyvalent) ion.

† The same adjectives are also used in medicine to refer to vaccine valence, with the slight difference that in the latter sense, quadri- is more common than tetra-.

The oxidation state of an atom in a molecule gives the number of valence electrons it has gained or lost.

[20][21] The International Union of Pure and Applied Chemistry (IUPAC) has made several attempts to arrive at an unambiguous definition of valence.

The current version, adopted in 1994:[22] Hydrogen and chlorine were originally used as examples of univalent atoms, because of their nature to form only one single bond.

Hydrogen has only one valence electron and can form only one bond with an atom that has an incomplete outer shell.