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Valența caracterizează capacitatea de combinare a unui atom cu un alt atom. Ea este dată de numărul electronilor cu care atomul participă la formarea legăturilor chimice și variază în funcție de atom și grupare chimică.

Valența este determinată de numărul electronilor care participă la legătura chimică. Numărul grupei principale indică valența maximă a elementelor. Valența se poate raporta la hidrogen sau la oxigen. Valența elementelor raportată la hidrogen crește în perioadă de la grupa I-A la grupa IV-A, apoi scade. Pentru elementele din grupele V-A, VI-A și VII-A valența față de hidrogen se stabilește cu ajutorul relației: Valența = 8 - nr. grupei principale. Valența maximă a elementelor față de oxigen este egală cu numărul grupei principale (excepție făcând fluorul, care este constant monovalent). Nemetalele din grupele V-A, VI-A și VII-A pot avea și valențele cu două unitați mai mici decât valoarea maximă față de oxigen. Elementele din grupa VIII-A, grupa gazelor rare (nobile), nu au valență. Electrovalența reprezintă valența elementelor care se transformă ușor în ioni și este egală cu numărul de electroni cedați sau acceptați. Covalența reprezintă valența exprimată prin numărul de electroni pe care un atom îi pune în comun cu electronii altui atom. Covalența se notează cu cifre romane scrise în paranteză în dreapta simbolului chimic. De exemplu: H(I), O(II).

Conceptul de valență a fost dezvoltat în cea de-a 2-a jumătate a secolului 19, fiind folosit cu succes în explicarea structurii moleculare ale compușilor organici și anorganici.[1] The quest for the underlying causes of valence led to the modern theories of chemical bonding, including Lewis structures (1916), valence bond theory (1927), molecular orbitals (1928), valence shell electron pair repulsion theory (1958), and all of the advanced methods of quantum chemistry.

Valența elementelor se consideră față de două elemente:

Descriere[modificare | modificare sursă]

The combining power or affinity of an atom of an element was determined by the number of hydrogen atoms that it combined with. In methane, carbon has a valence of 4; in ammonia, nitrogen has a valence of 3; in water, oxygen has a valence of two; and in hydrogen chloride, chlorine has a valence of 1. Chlorine, as it has a valence of one, can be substituted for hydrogen, so phosphorus has a valence of 5 in phosphorus pentachloride, PCl5. Valence diagrams of a compound represent the connectivity of the elements, with lines drawn between two elements, sometimes called bonds, representing a saturated valency for each element.[1] Examples are

Compound H2 CH4 C3H8 C2H2 NH3 NaCN H2S H2SO4 Cl2O7
Diagram Wasserstoff.svg Methane-2D-flat-small.png Propane Lewis.svg Acetylene-2D.png
Sodium cyanide-2D.svg Hydrogen sulfide.svg Sulfuric acid chemical structure.png Dichlorine heptoxide.svg
Valencies Hydrogen 1 Carbon 4
Hydrogen 1
Carbon 4
Hydrogen 1
Carbon 4
Hydrogen 1
Nitrogen 3
Hydrogen 1
Sodium 1
Carbon 4
Nitrogen 3
Sulfur 2
Hydrogen 1
Sulfur 6
Oxygen 2
Hydrogen 1
Chlorine 7
Oxygen 2

Valence only describes connectivity; it does not describe the geometry of molecular compounds, or what are now known to be ionic compounds or giant covalent structures. A line between atoms does not represent a pair of electrons as it does in Lewis diagrams.

Modern definitions[modificare | modificare sursă]

Valence is defined by the IUPAC as:-

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. .[2]

An alternative modern description is:-[3]

The number of hydrogen atoms that can combine with an element in a binary hydride or twice the number of oxygen atoms combining with an element in its oxide or oxides. This definition differs from the IUPAC definition as an element can be said to have more than one valence.

Etimologie[modificare | modificare sursă]

The etymology of the word "valence" 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.[4]

William Higgins' combinations of ultimate particles (1789)

In 1789, William Higgins published views on what he called combinations of "ultimate" particles, which foreshadowed the concept of valency bonds.[5] 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 (see illustration).

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[6]

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).[5] In 1857 August Kekulé proposed fixed valences for many elements, such as four for carbon, and used them to propose structural formulas for many organic molecules which are still accepted today.

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 [Co(NH3)6]Cl3 in which he distinguished primary and secondary 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 (maximum and minimum oxidation states), and proposed Abegg's rule to the effect that their difference is often eight.

Electronii și valența[modificare | modificare sursă]

The Rutherford model of the nuclear atom (1911) 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 (main-group) atoms to achieve a stable octet of eight valence-shell electrons. According to Lewis, covalent bonding leads to octets by sharing of electrons, and ionic bonding leads to octets by 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."[7] The prefix co- means "together", so that a co-valent bond means that the atoms share valence. Subsequent to this, 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.

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 maximum of four allowed by the octet rule. For example, in the sulfur hexafluoride molecule (SF6), Pauling considered that the sulfur forms six true two-electron bonds using so-called 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 SF6 molecule should be described as having six polar covalent (partly ionic) bonds made from only four orbitals on sulfur (one s and three p) in accordance with the octet rule, together with six orbitals on the fluorines.[8] Similar calculations on transition metal molecules show that the role of p orbitals is minimal, so that at most one s and five d orbitals on the metal participate in bonding.

Valori ale valenței[modificare | modificare sursă]

For elements in the main groups of the periodic table, the valence can vary between one and seven.

Group Valence 1 Valence 2 Valence 3 Valence 4 Valence 5 Valence 6 Valence 7 Typical valencies
1 (I) NaCl 1
2 (II) MgCl2 2
13 (III) BCl3, AlCl3
14 (IV) CO CH4 4
15 (V) NO NH3
NO2 N2O5
3 and 5
16 (VI) H2O
SO2 SO3 2 and 6
17 (VII) HCl ClO2 Cl2O7 1 and 7

Many elements have a common valence related to their position in the periodic table, and nowadays this is rationalised by the octet rule. The Latin/Greek prefixes uni/mono, bi/di, ter/tri, quadri/tetra, quinque/penta are used to describe ions in the one, two, three, four or five charge states. Polyvalence or multivalence refers to species that are not restricted to a specific number of valence bonds. Species with a single charge are univalent (monovalent)). For example, the Cs+ cation is a univalent or monovalent cation, whereas the Ca2+ cation is a divalent cation, and the Fe3+ 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 (polyvalent) ion.[necesită citare]

Valența și numărul de oxidare[modificare | modificare sursă]

Because of the ambiguity of the term valence,[9] nowadays other notations are used in practice. Beside the system of oxidation numbers as used in Stock nomenclature for coordination compounds,[10] and the lambda notation, as used in the IUPAC nomenclature of inorganic chemistry,[11] "oxidation state" is a more clear indication of the electronic state of atoms in a molecule.

The "oxidation state" of an atom in a molecule gives the number of valence electrons it has gained or lost.[12] In contrast to the valency number, the oxidation state can be positive (for an electropositive atom) or negative (for an electronegative atom).

Elements in a high oxidation state can have a valence higher than four. For example, in perchlorates, chlorine has seven valence bonds and ruthenium, in the +8 oxidation state in ruthenium tetroxide, has eight valence bonds.

Exemple[modificare | modificare sursă]

(valencies according to the number of valence bonds definition and conform oxidation states)

Hydrogen chloride HCl H = 1   Cl = 1 H = +1   Cl = −1
Perchloric acid * HClO4 H = 1   Cl = 7   O = 2 H = +1   Cl = +7   O = −2
Sodium hydride NaH Na = 1   H = 1 Na = +1   H = −1
Ferrous oxide ** FeO Fe = 2   O = 2 Fe = +2   O = −2
Ferric oxide ** Fe2O3 Fe = 3   O = 2 Fe = + 3   O = −2

* The univalent perchlorate ion (ClO4) has valence 1.
** Iron oxide appears in a crystal structure, so no typical molecule can be identified.
  In ferrous oxide, Fe has oxidation number II, in ferric oxide, oxidation number III.

Examples where valences and oxidation states differ due to bonds between identical atoms:

Chlorine Cl2 Cl = 1 Cl = 0
Hydrogen peroxide H2O2 H = 1   O = 2 H = +1   O = −1
Acetylene C2H2 C = 4   H = 1 C = −1   H = +1
Mercury(I) chloride Hg2Cl2 Hg = 2   Cl = 1 Hg = +1   Cl = −1

Valences may also be different from absolute values of oxidation states due to different polarity of bonds. For example, in dichloromethane, CH2Cl2, carbon has valence 4 but oxidation state 0.

Definirea "numărului maxim de legături"[modificare | modificare sursă]

Frankland took the view that the valence (he used the term "atomicity") of an element was a single value that corresponded to the maximum value observed. The number of unused valencies on atoms of what are now called the p-block elements is generally even, and Frankland suggested that the unused valencies saturated one another. For example, nitrogen has a maximum valence of 5, in forming ammonia two valencies are left unattached; sulfur has a maximum valence of 6, in forming hydrogen sulphide four valencies are left unattached.[13][14]

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:[15]

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.[2]

Hydrogen and chlorine were originally used as examples of univalent atoms, because of their nature to form only one single bond. Hydrogen has only one valence electron and can form only one bond with an atom that has an incomplete outer shell. Chlorine has seven valence electrons and can form only one bond with an atom that donates a valence electron to complete chlorine's outer shell. However, chlorine can also have oxidation states from +1 to +7 and can form more than one bond by donating valence electrons.

Although hydrogen has only one valence electron, it can form bonds with more than one atom. In the bifluoride ion ([HF2]-), for example, it forms a three-center four-electron bond with two fluoride atoms:

[ F–H F ↔ F H–F ]

Another example is the Three-center two-electron bond in diborane (B2H6).

Maximum valences of the elements[modificare | modificare sursă]

Maximum valences for the elements are based on the data from list of oxidation states of the elements.

References[modificare | modificare sursă]

  1. ^ a b Partington, James Riddick (1921). A text-book of inorganic chemistry for university students (ed. 1st). Accesat la 13 aprilie 2014 
  2. ^ a b IUPAC Gold Book definition: valence
  3. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (ed. 2nd). Oxford: Butterworth-Heinemann. ISBN 0080379419 
  4. ^ Format:OEtymD
  5. ^ a b Partington, J.R. (1989). A Short History of Chemistry. Dover Publications, Inc. ISBN 0-486-65977-1 
  6. ^ Frankland, E. (1852). Phil. Trans., vol. cxlii, 417.
  7. ^ Langmuir, Irving (1919). „The Arrangement of Electrons in Atoms and Molecules”. Journal of the American Chemical Society 41 (6): 868–934. doi:10.1021/ja02227a002. 
  8. ^ E. Magnusson. Hypercoordinate molecules of second-row elements: d functions or d orbitals? J. Am. Chem. Soc. 1990, 112, 7940–7951. doi:10.1021/ja00178a014
  9. ^ The Free Dictionary: valence
  10. ^ IUPAC, Gold Book definition: oxidation number
  11. ^ IUPAC, Gold Book definition: lambda
  12. ^ IUPAC Gold Book definition: oxidation state
  13. ^ Frankland, E. (1870). Lecture notes for chemical students(Google eBook) (ed. 2d). J. Van Voorst. p. 21. 
  14. ^ Frankland, E.; Japp, F.R (1885). Inorganic chemistry (ed. 1st). pp. 75–85. Accesat la 8 aprilie 2014 
  15. ^ Muller, P. (1994). „Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)”. Pure and Applied Chemistry 66 (5). doi:10.1351/pac199466051077.