1 used of a chromosome that is not paired or united with its homologous chromosome during synapsis; "a univalent chromosome" [ant: bivalent, multivalent]
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.
HistoryThe etymology of the word "valence" is from 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.
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 similarly 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 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 Franklin, that their affinities are best satisfied. Following these examples and postulates, Franklin declares how obvious it is that:
This “combining power” was afterwards called quantivalence or valency (and valence by American chemists).:
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 elementsValences for the majority of elements are based on the highest known fluoride.
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.
univalent in Afrikaans: Valensie
univalent in Arabic: تكافؤ
univalent in Bosnian: Valencija (hemija)
univalent in Bulgarian: Валентност
univalent in Catalan: València (química)
univalent in Danish: Valens
univalent in German: Wertigkeit (Chemie)
univalent in Spanish: Valencia atómica
univalent in Esperanto: Valento
univalent in Basque: Balentzia
univalent in French: Valence (chimie)
univalent in Galician: Valencia atómica
univalent in Italian: Valenza
univalent in Hebrew: ערכיות
univalent in Georgian: ვალენტობა
univalent in Lithuanian: Valentingumas
univalent in Latvian: Vērtība (ķīmija)
univalent in Dutch: Valentie (chemie)
univalent in Japanese: 原子価
univalent in Polish: Wartościowość
univalent in Portuguese: Valência (química)
univalent in Russian: Валентность
univalent in Simple English: Valence (chemistry)
univalent in Finnish: Valenssi (kemia)
univalent in Vietnamese: Hóa trị
univalent in Ukrainian: Валентність
univalent in Urdu: ظرف (کیمیاء)
univalent in Chinese: 化合价