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valence

  

(vā'ləns) pronunciation also valency (-lən-sē)

n., pl. -lenc·es also -len·cies.

  1. Chemistry.
    1. The combining capacity of an atom or radical determined by the number of electrons that it will lose, add, or share when it reacts with other atoms.
    2. A positive or negative integer used to represent this capacity: The valences of copper are 1 and 2.
  2. The number of binding sites of a molecule, such as an antibody or antigen.
  3. The ability of a substance to interact with another or to produce an effect.
  4. Psychology. The degree of attraction or aversion that an individual feels toward a specific object or event.
  5. Linguistics. The number of arguments that a lexical item, especially a verb, can combine with to make a syntactically well-formed sentence, often along with a description of the categories of those constituents. Intransitive verbs (appear, arrive) have a valence of one—the subject; some transitive verbs (paint, touch), two—the subject and direct object; other transitive verbs (ask, give), three—the subject, direct object, and indirect object.
  6. The capacity of something to unite, react, or interact with something else: “I do not claim to know much more about novels than the writing of them, but I cannot imagine one set in the breathing world which lacks any moral valence” (Robert Stone).

[Latin valentia, capacity, from valēns, valent-, present participle of valēre, to be strong.]


A term commonly used by chemists to characterize the combining power of an element for other elements, as measured by the number of bonds to other atoms which one atom of the given element forms upon chemical combination. The term also has come to signify the theory of all the physical and chemical properties of molecules that specially depend on molecular electronic structure.

Thus, in water, H2O, the valence of each hydrogen atom is 1; the valence of oxygen, 2. In methane, CH4, the valence of hydrogen again is 1; of carbon, 4. In NaCl and CCl4 the valence of chlorine is 1, and in CH2 the valence of carbon is 2. See also Chemical bonding.

Most of the simple facts of valence (though certainly not all) follow from the postulate that atoms combine in such a way as to seek closed-shell or inert-gas structures (rule of eight) by the transfer of electrons between them or the sharing of a pair of electrons between them. Many molecular structures may be obtained by inspection using these rules; letting a dot represent an electron: In these electron-dot symbols, the electrons in the K shell are not included for atoms after He, nor are the electrons in the K and L shells for atoms following Ne.

As generally used and here defined, the word valence is ambiguous. Before a value can be assigned to the valence of an atom in a molecule, the electronic structure of the molecule must be exactly known, and this structure must be describable simply in terms of simple bonds. In practice neither of these conditions is ever precisely fulfilled. A term not so ambiguous is oxidation number or valence number. Oxidation numbers are useful for the balancing of oxidation-reduction equations, but they are not related simply to ordinary valences. Thus the valence of carbon in CH4, CHCl3, and CCl4 is 4; oxidation numbers of carbon in these three substances are −4, +2, and +4. See also Electronegativity; Molecular orbital theory; Oxidation-reduction.

Number of bonds (see bonding) an atom can form. Hydrogen (H) always has valence 1, so other elements' valences equal the number of hydrogen atoms they combine with. Thus, oxygen (O) has valence 2, as in water (H2O); nitrogen (N) has valence 3, as in ammonia (NH3); and chlorine (Cl) has valence 1, as in hydrochloric acid (HCl). The valence depends on the number of unpaired electrons in the outermost (and, in transition elements, the next) shell of the atom's structure. The sharing of the unpaired (valence) electrons in a bond mimics the stable configuration of the noble gases, whose outer shells are full. Elements that can achieve stable configurations by various combinations have more than one valence.

For more information on valence, visit Britannica.com.

combining capacity of an atom expressed as the number of single bonds the atom can form or the number of electrons an element gives up or accepts when reacting to form a compound. Atoms are called monovalent, divalent, trivalent, or tetravalent, according to whether they form one, two, three, or four bonds (see chemical bond).

The Relationship of Electrons and Valence

For purposes of describing chemical behavior, an atom can be considered as a positively charged nucleus surrounded by negatively charged electrons orbiting in concentric spherical shells. The number of positive charges in the nucleus determines how many electrons normally surround the nucleus; as atomic number increases, the electron shells are filled, starting with those nearest the nucleus.

The valence of an atom is determined by the number of electrons in the outermost, or valence, shell. The atom exists in its most stable configuration when its outermost shell is completely filled; in combining with other atoms, it thus tends to gain or lose valence electrons in order to attain a stable configuration. If the valence shell of the atom is nearly complete, as in chlorine and other nonmetals, the atom will tend to accept electrons to complete it; if the valence shell has few electrons, as in potassium and other metals, the atom will tend to lose these electrons, so that the next shell below the valence shell becomes a completed outermost shell.

The valence of many elements is determined from their ability to combine with hydrogen or to replace it in compounds. For example, one oxygen atom combines with two hydrogen atoms to form water and the valence of oxygen is thus determined to be 2. Similarly, chlorine accepts one electron in combining with a single atom of hydrogen to form hydrogen chloride, HCl, and chlorine's valence is 1. Zinc does not combine with hydrogen but does replace it in compounds; in a typical replacement reaction, one zinc atom replaces two hydrogen atoms, as in the equation Zn+H2SO4→ZnSO4+H2, so that zinc has a valence of 2.

Valence Number

Atoms are assigned numbers, called valence numbers, oxidation numbers, or oxidation states, which range in value from −4 through 0 to +7 and describe the combining behavior of the atoms in chemical reactions, particularly oxidation-reduction reactions (see oxidation and reduction). Metals, which commonly donate electrons and form compounds in which they exist in the positive, or cationic, state, are assigned positive oxidation numbers (see cation). For a metal such as zinc, which donates two electrons to achieve a stable electron configuration, the oxidation number is +2. Nonmetals, which commonly accept electrons and in compounds exist in the negative, or anionic, state, are assigned negative oxidation numbers (see anion). The oxidation number is −1 for chlorine and the other halogens, which accept one electron to complete their valence shell.

Some elements, like the transition metals, have electron configurations in which electrons from their inner shells can also be used as valence electrons; these elements can have several different oxidation states. For example, iron can have a valence of +2 or +3, and chromium can have a valence of +2, +3, or +6. Iron in the +3 oxidation state, Fe+3, acts as an oxidizing agent, accepting one electron to attain the Fe+2 state, while ferrous iron, Fe+2, by donating an electron in going to the +3 state, acts as a reducing agent.

The relative combining capacity of an atom in relation to hydrogen, which has a valence of 1. Some elements, such as iron, sometimes have more than one valence (ferrous, Fe +2 , and ferric, Fe +3 ), so that you can get either FeO, or Fe 2 O 3 .


Dansk (Danish)
1.
n. - valens

idioms:

  • valence electron    valenselektron

2.
n. - flæse, gardinkappe, hyldebort, omhæng, skærmkant

Nederlands (Dutch)
verenigingskracht van atoom, reactiekracht (biologie), aantrekkingskracht

Français (French)
1.
n. - (Chim, Ling) valence

idioms:

  • valence electron    électron de valence

2.
n. - tour de lit, lambrequin, cantonnière

Deutsch (German)
1.
n. - Wertigkeit, Valenz

idioms:

  • valence electron    Valenzelektron

2.
n. - kurzer Behang, Volant

Ελληνική (Greek)
n. - (βιολ., χημ.) σθένος

idioms:

  • valence electron    ηλεκτρόνιον σθένους

Italiano (Italian)
valenza, drappeggio

idioms:

  • valence electron    elettrone di valenza

Português (Portuguese)
n. - valência (f) (Quím.) (Biol.)

idioms:

  • valence electron    elétron de valência

Русский (Russian)
валентность, лекарство из трав, доблесть

idioms:

  • valence electron    валентный электрон

Español (Spanish)
1.
n. - valencia

idioms:

  • valence electron    electrón de valencia

2.
n. - cenefa, doselera

Svenska (Swedish)
n. - valens, atomvärde (tekn.)

中文（简体） (Chinese (Simplified))
价, 原子价

idioms:

  • valence electron    原子价电子

中文（繁體） (Chinese (Traditional))
n. - 價, 原子價

idioms:

  • valence electron    原子價電子

한국어 (Korean)
1.
n. - (화학) 원자가, (염색체의 결합) 수가, 유의성

2.
n. - 밸런스, 드리운 천

日本語 (Japanese)
n. - 原子価, 結合価, ヴァランス, 掛け布

idioms:

  • valence electron    価電子

العربيه (Arabic)
‏(الاسم) وحدة قدرة اتحاد الذرات‏

עברית (Hebrew)
n. - ‮איות אחר למילה YNCELAV‬
n. - ‮איות אחר למילה ECNALAV‬

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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved.  Read more
Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved.  Read more
Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/  Read more
Science Dictionary. The New Dictionary of Cultural Literacy, Third Edition Edited by E.D. Hirsch, Jr., Joseph F. Kett, and James Trefil. Copyright © 2002 by Houghton Mifflin Company. Published by Houghton Mifflin. All rights reserved.  Read more
Geological Glossary. Peterson Field Guide to Rocks and Minerals, by Frederick H. Pough. Copyright © 1998 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.  Read more
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