Valence (chemistry)

In chemistry, 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. Over the last century, the concept of valence evolved into a range of approaches for describing the chemical bond, including Lewis structures (1916), valence bond theory (1927), molecular orbitals (1928), valence shell electron pair repulsion theory (1958) and all the advanced methods of quantum chemistry.

Contents

History

The etymology of the word "valence" traces back to 1425, meaning "extract, preparation," from Latin valentia "strength, capacity," and the chemical meaning referring to the "combining power of an element" is recorded from 1884, from German Valenz.[1]

In 1789, William Higgins published views on what he called combinations of "ultimate" particles, which foreshadowed the concept of valency bonds.[2] If, for example, according to Higgins, the force between the ultimate particle of oxygen and the ultimate particle of nitrogen were 6, then the strength of the force would be divided accordingly, and similarly for the other combinations of ultimate particles:

William Higgins' combinations of ultimate particles (1789)

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 theories of free radicals and “type 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 three atom groups (e.g. NO3, NH3, NI3, etc.) or 5, i.e. in the five atom groups (e.g. NO5, NH4O, PO5, etc.), equivalents of the attached elements. It is in this manner, according to Frankland, that their affinities are best satisfied. Following these examples and postulates, Frankland declares how obvious it is that:[3]

“ 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).[2]

Overview

The concept was developed in the middle of the nineteenth century in an attempt to rationalize the formulae of different chemical compounds. In 1919, Irving Langmuir, borrowed the term to explain Gilbert N. Lewis's cubical atom model by stating that "the number of pairs of electrons which any given atom shares with the adjacent atoms is called the covalence of that atom." The prefix co- means "together", so that a co-valent bond means that the atoms share valence. Hence, if an atom, for example, had a +1 valence, meaning it was missing an electron, and another a −1 valence, meaning it had an extra electron, then a bond between these two atoms would result because they would be complementing or sharing their out of balance valence tendencies. Subsequently, it is now more common to speak of covalent bonds rather than "valence", which has fallen out of use in higher level work with the advances in the theory of chemical bonding, but is still widely used in elementary studies where it provides a heuristic introduction to the subject.

"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 many 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,[4]:

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.[5]

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 bifluoride ion, [HF2]−, and the various boron hydrides such as diborane: these are examples of three-center 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

Maximum valences for the majority of elements are based on the highest known fluoride.[6] ex. Hydrogen's valency is 1.

Group → 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
↓ Period
1 1
H
2
He
2 3
Li		 4
Be
5
B		 6
C		 7
N		 8
O		 9
F		 10
Ne
3 11
Na		 12
Mg
13
Al		 14
Si		 15
P		 16
S		 17
Cl		 18
Ar
4 19
K		 20
Ca		 21
Sc		 22
Ti		 23
V		 24
Cr		 25
Mn		 26
Fe		 27
Co		 28
Ni		 29
Cu		 30
Zn		 31
Ga		 32
Ge		 33
As		 34
Se		 35
Br		 36
Kr
5 37
Rb		 38
Sr		 39
Y		 40
Zr		 41
Nb		 42
Mo		 43
Tc		 44
Ru		 45
Rh		 46
Pd		 47
Ag		 48
Cd		 49
In		 50
Sn		 51
Sb		 52
Te		 53
I		 54
Xe
6 55
Cs		 56
Ba		 *
 72
Hf		 73
Ta		 74
W		 75
Re		 76
Os		 77
Ir		 78
Pt		 79
Au		 80
Hg		 81
Tl		 82
Pb		 83
Bi		 84
Po		 85
At		 86
Rn
7 87
Fr		 88
Ra		 **
 104
Rf		 105
Db		 106
Sg		 107
Bh		 108
Hs		 109
Mt		 110
Ds		 111
Rg		 112
Cn		 113
Uut		 114
Uuq		 115
Uup		 116
Uuh		 117
Uus		 118
Uuo

* Lanthanides 57
La		 58
Ce		 59
Pr		 60
Nd		 61
Pm		 62
Sm		 63
Eu		 64
Gd		 65
Tb		 66
Dy		 67
Ho		 68
Er		 69
Tm		 70
Yb		 71
Lu
** Actinides 89
Ac		 90
Th		 91
Pa		 92
U		 93
Np		 94
Pu		 95
Am		 96
Cm		 97
Bk		 98
Cf		 99
Es		 100
Fm		 101
Md		 102
No		 103
Lr

Valences of chemical elements

     None      One      Two      Three      Four      Five      Six      Seven      Eight      Nine
Atomic number colors show state at standard temperature and pressure (0 °C and 1 atm)
Solids Liquids Gases Unknown
Borders show natural occurrence
Primordial From decay Synthetic (Undiscovered)

Other criticisms of the concept of valence

See also

References

  1. ↑ Valence - Online Etymology Dictionary.
  2. ↑ 2.0 2.1 Partington, J.R. (1989). A Short History of Chemistry. Dover Publications, Inc. ISBN 0-486-65977-1. 
  3. ↑ Frankland, E. (1852). Phil. Trans., vol. cxlii, 417.
  4. ↑ Pure Appl. Chem. 66: 1175 (1994).
  5. ↑ IUPAC Gold Book: valence  PDF:PDF (2.70 KiB) from the IUPAC Gold Book
  6. ↑ http://www.webelements.com/ (accessed 2006-02-20).
  7. ↑ 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 ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5 
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