Valence (chemistry)

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For the mathematical definition, see Univalent function.

Valence, also known as valency or valency number, is a measure of the number of chemical bonds formed by the atoms of a given element. The concept was developed in the middle of the nineteenth century in an attempt to rationalize the formulae of different chemical compounds. Although it has fallen out of use in higher level work with the advances in the theory of chemical bonding, it is still widely used in elementary studies where it provides a heuristic introduction to the subject.

Contents

1 \"Number of bonds\" definition
2 IUPAC definition
3 Valences of the elements
4 Other criticisms of the concept of valence
5 See also
6 References
7 External links

\"Number of bonds\" definition

The number of bonds formed by a given element was originally thought to be a fixed chemical property and in fact, in many cases, this is a good approximation. For example, in virtually all of their compounds, carbon forms four bonds, oxygen two and hydrogen one. However it soon became apparent that, for many elements, the valence could vary between different compounds. One of the first examples to be identified was phosphorus, which sometimes behaves as if it has a valence of three and sometimes as if it has a valence of five. One method around this problem is to specify the valence for each individual compound: although it removes much of the generality of the concept, this approach has given rise to the idea of oxidation numbers (used in Stock nomenclature) and to lambda notation in the IUPAC nomenclature of inorganic chemistry.

IUPAC definition

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,^{[#endnote_PAC]} is:

The maximum number of univalent atoms (originally hydrogen or chlorine atoms) that may combine with an atom of the element under consideration, or with a fragment, or for which an atom of this element can be substituted.

This definition reimposes a unique valence for each element at the expense of neglecting, in many cases, a large part of its chemistry.

The mention of hydrogen and chlorine is for historic reasons, although both in practice mostly form compounds in which their atoms form a single bond. Exceptions in the case of hydrogen include the ion [HF₂]⁻ and the various boron hydrides such as diborane: these are examples of three-center two-electron bonds. Chlorine forms a number of fluorides—ClF, ClF₃ and ClF₅—and its valence according to the IUPAC definition is hence five. Fluorine is the element for which the largest number of atoms combine with atoms of other elements: it is univalent in all compounds except the ion [H₂F]⁺. In fact, the IUPAC definition can only be resolved by fixing the valences of hydrogen and fluorine as one, a convention which has been followed here.

Valences of the elements

Valences for the majority of elements are based on the highest known fluoride.^{[#endnote_Table_ref]}

Other criticisms of the concept of valence

The valence of an element is not always equal to its highest oxidation state: exceptions include ruthenium, osmium and xenon, which have valences of six (hexafluorides) but which form compounds with oxygen in the +8 oxidation state, and chlorine, which has a valence of five but a highest oxidation state of +7 (in perchlorates).
The concept of "combination" cannot be equated with the number of bonds formed by an atom. In lithium fluoride (which has the NaCl structure), each lithium atom is surrounded by six fluorine atoms, whereas the valence of lithium is universally taken to be one, as the formula LiF would suggest.^{[#endnote_LiF_note]}

References

↑ Pure Appl. Chem. 66: 1175 (1994).
↑ http://www.webelements.com/ (accessed 2006-02-20).
↑ In the gas phase, LiF does indeed exist as discrete diatomic molecules as the valences would suggest: Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th Edn.) New York:Wiley-Interscience. ISBN 0-471-19957-5.

External links

["Valence"] from the IUPAC Gold Book (PDF file)

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