Valence (chemistry)

See also: Valence

not to confuse Layer of valence and Valence

In chemistry, the valence of a element is the maximum number of chemical bonds which it can form. Or, in other words, it is the maximum number of atoms of a univalent element which an atom of this element can join. The valence of an atom within the framework of a Molécule or a Ion is the number of covalent bonds that this atom formed. The valence of a monoatomic Ion in solution is its load, one speaks then about electrovalence.

The Couche of valence is the last electron shell of an atom, i.e. that which is taken into account to determine the valence .

Nomenclature

  • Valence 1 : plain is worth

  • Valence 2 : Bi is worth or di is worth
  • Valence 3 : tri is worth
  • Valence 4 : will tétra are worth
  • Valence 5 : penta is worth
  • Valence 6 : hexa is worth
  • Valence 7 : hepta is worth

Valence of the elements

Principle

An atom tends to being stable when its last layer of electrons (Couche of valence) is complete, i.e. that it contains the maximum number of electrons which it can contain. What means that the atom will tend to want to acquire electrons, to supplement this layer, or to yield some, to empty its layer of valence, and to be with the complete sub-base. (see also the Rule of the byte).

The Rare gas S (He, , Ar, Kr, Xe, RN), have a layer of naturally complete valence, and thus do not link themselves with other atoms: their valence is 0. For this reason, one finds them in nature in the monoatomic gas shape.

Elementary examples

An atom of Hydrogène H can form only one chemical bond with another atom. Its valence is thus 1, it is a univalent element . Thus, in the molecule of Dihydrogène H2 , two atoms H are bound by a Covalent bond.

The atom of Oxygen O with the capacity to join two atoms, and thus has a valence of 2: it is a bivalent element or divalent . In the molecule of Water, H2O , the oxygen atom forms two covalent bonds, one with each hydrogen atom. Each one of the latter, on the other hand, forms only one connection.

An atom of Carbone C is tetravalent , and can form up to 4 chemical bonds. Thus in the molecule of Methane CH4 , central carbon is related to 4 hydrogen.

Valence of the atoms in the molecules

Within the framework of a Molecule, the valence of a Atome is the number of bonds which this atom formed. For example, although the valence of carbon is 4, in the case of a Carbène (R2C:), the valence of carbon is of 2.

Several elements exist in different steps oxidation. In the case of the nonmetal elements , the states of oxidation are related to the valence. Thus, phosphorus has the states of oxidation +III and +V (it can metttre concerned 3 or 5 electrons in polarized covalent bonds). In a similar way, one speaks about pentavalent phosphorus (5 connections) and about trivalent phosphorus (three connections).

Case of the multiple connections

A connection can be double, triple or quadruple. For example in the case of the Carbon dioxide, CO2 , each of the two oxygen atoms forms a double covalent bond with central carbon. The formula of the molecule is: O=C=O. in this molecule carbon is tetravalent (two double linkings) and divalent oxygen (a connection doubles)

See too

Simple: Valence (chemistry)

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