Definition - What does Valence mean?
In chemistry, the valence of an element is a measure of its combining power with other atoms when it forms chemical compounds or molecules.
It is the quality that determines the number of atoms or groups with which any single atom or group can unite chemically. It is the relative combining capacity of an atom or group compared with that of the standard hydrogen atom. The chloride ion, Cl-, with a valence of one, has the capacity to unite with one atom of hydrogen or its equivalent, as in hydrogen chloride or sodium chloride.
Valence is also known as valency or oxidation state.
Corrosionpedia explains Valence
Valence is a whole number (positive or negative) representing the power of one element to combine with another. In general terms, the valence number represents the number of electrons in an atom or combined group of atoms which can be easily given up or accepted to react with or bond to another atom or group of atoms to form a molecule.
Because of the ambiguity of the term "valence," "oxidation state" is commonly used. The oxidation state of an atom in a molecule gives the number of valence electrons it has gained or lost.
When considering the electronic configuration of an atom, electrons can be divided into two categories, according to their positions in an atom: valence and core electrons. Valence electrons are those occupying the outermost shell or highest energy level of an atom, while core electrons are those occupying the innermost shell or lowest energy level. Generally, valence electrons can participate in the formation of chemical bonding, but core electrons cannot. The number of valence electrons is critical to the chemical property of an atom.
The chemical reactivity of an atom is mainly determined by valence electrons. Atoms which have a complete shell of valence electrons tend to be chemically inert. Atoms with one or two valence electrons are highly reactive. This phenomenon can be explained by Hund's rule, which states that an orbital that is empty, half-full or full are more stable than those that are not.
Valence electrons are also responsible for the electrical conductivity of an element; as a result, an element may be classified as a metal, nonmetal or semiconductor (metalloid).