Valence (chemistry)


In chemistry, the valence or valency of an atom is a measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Valence is generally understood to be the number of chemical bonds that each atom of a given chemical element typically forms. Double bonds are considered to be two bonds, triple bonds to be three, quadruple bonds to be four, quintuple bonds to be five and sextuple bonds to be six. In most compounds, the valence of hydrogen is 1, of oxygen is 2, of nitrogen is 3, and of carbon is 4. Valence is not to be confused with the related concepts of the coordination number, the oxidation state, or the number of valence electrons for a given atom.

Description

The valence is the combining capacity of an atom of a given element, determined by the number of hydrogen atoms that it combines with. In methane, carbon has a valence of 4; in ammonia, nitrogen has a valence of 3; in water, oxygen has a valence of 2; and in hydrogen chloride, chlorine has a valence of 1. Chlorine, as it has a valence of one, can be substituted for hydrogen in many compounds. Phosphorus has a valence 3 in phosphine and a valence of 5 in phosphorus pentachloride, which shows that an element may exhibit more than one valence. The structural formula of a compound represents the connectivity of the atoms, with lines drawn between two atoms to represent bonds. The two tables below show examples of different compounds, their structural formulas, and the valences for each element of the compound.
Compound
Hydrogen
Methane
Propane
Propylene
Acetylene
Diagram
Valencies

Compound
Ammonia		NaCN
Sodium cyanide
Thiophosphoryl chloride
Hydrogen sulfide
Sulfuric acid
Dithionic acid
Dichlorine heptoxide
Xenon tetroxide
Diagram
Valencies

Definition

Valence is defined by the IUPAC as:
An alternative modern description is:
This definition differs from the IUPAC definition as an element can be said to have more than one valence.

Historical development

The etymology of the words valence and valency traces back to 1425, meaning "extract, preparation", from Latin valentia "strength, capacity", from the earlier valor "worth, value", and the chemical meaning referring to the "combining power of an element" is recorded from 1884, from German Valenz.
Image:Higgins-particles.jpg|350px|thumb|William Higgins' combinations of ultimate particles
The concept of valence was developed in the second half of the 19th century and helped successfully explain the molecular structure of inorganic and organic compounds.
The quest for the underlying causes of valence led to the modern theories of chemical bonding, including the cubical atom, Lewis structures, valence bond theory, molecular orbitals, valence shell electron pair repulsion theory, and all of the advanced methods of quantum chemistry.
In 1789, William Higgins published views on what he called combinations of "ultimate" particles, which foreshadowed the concept of valency bonds. 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 likewise for the other combinations of ultimate particles.
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 radical 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 3-atom groups or 5, i.e., in the 5-atom groups, equivalents of the attached elements. According to him, this is the manner in which their affinities are best satisfied, and by following these examples and postulates, he declares how obvious it is that
This "combining power" was afterwards called quantivalence or valency. In 1857 August Kekulé proposed fixed valences for many elements, such as 4 for carbon, and used them to propose structural formulas for many organic molecules, which are still accepted today.
Lothar Meyer in his 1864 book, Die modernen Theorien der Chemie, contained an early version of the periodic table containing 28 elements, for the first time classified elements into six families by their valence. Works on organizing the elements by atomic weight, until then had been stymied by the widespread use of equivalent weights for the elements, rather than atomic weights.
Most 19th-century chemists defined the valence of an element as the number of its bonds without distinguishing different types of valence or of bond. However, in 1893 Alfred Werner described transition metal coordination complexes such as, in which he distinguished principal and subsidiary valences, corresponding to the modern concepts of oxidation state and coordination number respectively.
For main-group elements, in 1904 Richard Abegg considered positive and negative valences, and proposed Abegg's rule to the effect that their difference is often 8.
An alternative definition of valence, developed in the 1920's and having modern proponents, differs in cases where an atom's formal charge is not zero. It defines the valence of a given atom in a covalent molecule as the number of electrons that an atom has used in bonding:
or equivalently:
In this convention, the nitrogen in an ammonium ion bonds to four hydrogen atoms, but it is considered to be pentavalent because all five of nitrogen's valence electrons participate in the bonding.

Electrons and valence

The Rutherford model of the nuclear atom showed that the exterior of an atom is occupied by electrons, which suggests that electrons are responsible for the interaction of atoms and the formation of chemical bonds. In 1916, Gilbert N. Lewis explained valence and chemical bonding in terms of a tendency of atoms to achieve a stable octet of 8 valence-shell electrons. According to Lewis, covalent bonding leads to octets by the sharing of electrons, and ionic bonding leads to octets by the transfer of electrons from one atom to the other. The term covalence is attributed to Irving Langmuir, who stated in 1919 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 a valence. Subsequent to that, it is now more common to speak of covalent bonds rather than valence, which has fallen out of use in higher-level work from the advances in the theory of chemical bonding, but it is still widely used in elementary studies, where it provides a heuristic introduction to the subject.
In the 1930s, Linus Pauling proposed that there are also polar covalent bonds, which are intermediate between covalent and ionic, and that the degree of ionic character depends on the difference of electronegativity of the two bonded atoms.
Pauling also considered hypervalent molecules, in which main-group elements have apparent valences greater than the maximal of 4 allowed by the octet rule. For example, in the sulfur hexafluoride molecule, Pauling considered that the sulfur forms 6 true two-electron bonds using sp3d2 hybrid atomic orbitals, which combine one s, three p and two d orbitals. However more recently, quantum-mechanical calculations on this and similar molecules have shown that the role of d orbitals in the bonding is minimal, and that the molecule should be described as having 6 polar covalent bonds made from only four orbitals on sulfur in accordance with the octet rule, together with six orbitals on the fluorines. Similar calculations on transition-metal molecules show that the role of p orbitals is minor, so that one s and five d orbitals on the metal are sufficient to describe the bonding.

Common valences

For elements in the main groups of the periodic table, the valence can vary between 1 and 8.
GroupValence 1Valence 2Valence 3Valence 4Valence 5Valence 6Valence 7Valence 8Typical valences
1 NaCl
KCl		1
2 magnesium chloride|
Calcium chloride|		2
13 InBr
TlI		boron trichloride|
aluminium trichloride|
aluminium oxide|		3
14 CO
Lead chloride|		carbon dioxide|
methane|
Silicon tetrachloride|		2 and 4
15 NOammonia|
phosphine|
arsenic trioxide|		nitrogen dioxide|nitrogen pentoxide|
phosphorus pentachloride|		3 and 5
16 water|
hydrogen sulfide|
Sulfur dichloride|		sulfur dioxide|
Sulfur tetrafluoride|		sulfur trioxide|
Sulfur hexafluoride|
Sulfuric acid|		2, 4 and 6
17 HCl
ICl		chlorous acid|
Chlorine trifluoride|		chlorine dioxide|Iodine pentafluoride|
chloric acid|		Iodine heptafluoride|
dichlorine heptoxide|
perchloric acid|		1, 3, 5 and 7
18 Krypton difluoride|xenon tetrafluoride|xenon trioxide|xenon tetroxide|0, 2, 4, 6 and 8

Many elements have a common valence related to their position in the periodic table, and nowadays this is rationalised by the octet rule.
The Greek/Latin numeral prefixes are used to describe ions in the charge states 1, 2, 3, and so on, respectively. Polyvalence or multivalence refers to species that are not restricted to a specific number of valence bonds. Species with a single charge are univalent. For example, the cation is a univalent or monovalent cation, whereas the cation is a divalent cation, and the cation is a trivalent cation. Unlike Cs and Ca, Fe can also exist in other charge states, notably 2+ and 4+, and is thus known as a multivalent ion. Transition metals and metals to the right are typically multivalent but there is no simple pattern predicting their valency.
ValenceMore common adjective‡Less common synonymous adjective‡§
wikt:zerovalent#Adjective|wikt:nonvalent#Adjective|
wikt:monovalent#Adjective|wikt:univalent#Adjective|
wikt:divalent#Adjective|wikt:bivalent#Adjective|
wikt:trivalent#Adjective|wikt:tervalent#Adjective|
wikt:tetravalent#Adjective|wikt:quadrivalent#Adjective|
wikt:pentavalent#Adjective|wikt:quinquevalent#Adjective|, wikt:quinquivalent#Adjective|
wikt:hexavalent#Adjective|wikt:sexivalent#Adjective|
wikt:heptavalent#Adjective|wikt:septivalent#Adjective|
wikt:octavalent#Adjective|—
wikt:nonavalent#Adjective|—
wikt:decavalent#Adjective|—
undecavalent—
dodecavalent—
multiple / many / variablewikt:polyvalent#Adjective|wikt:multivalent#Adjective|
covalent bond|—
non-covalent interactions|—

† The same adjectives are also used in medicine to refer to vaccine valence, with the slight difference that in the latter sense, quadri- is more common than tetra-.
‡ As demonstrated by hit counts in Google web search and Google Books search corpora.
§ A few other forms can be found in large English-language corpora, but they are not the conventionally established forms in English and thus are not entered in major dictionaries.