Definition - What does Valence Number mean?
Valence Number is the number of valence 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. It is a whole number (positive or negative) which represents the power of one element to combine with another.
An example of valence number is as follows: 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. Here, the symbol Cl- represents 1 valence number or a valence electron needed by a chlorine atom to form a molecule. This Cl- symbol can also be written as Cl^(-1). Similarly, if any atom represents (+) symbol, for example sodium ion Na^(+2), this means that this ion has two valence electrons that it can give up to form a molecule.
Petropedia explains Valence Number
In chemistry, the valence of an element is a measure of its combining power with other atoms when it forms chemical compounds or molecules. 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 which occupy the outermost shell or highest energy level of an atom, while core electrons are those which occupy 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 while atoms with one or two valence electrons are highly reactive. 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).