Covalent bond
A covalent bond forming H2 (right) where two hydrogen atoms share the two electrons.

A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding.[1]

Covalent bonding includes many kinds of interaction, including σ-bonding, π-bonding, metal-to-metal bonding, agostic interactions, and three-center two-electron bonds.[2][3] The term covalent bond dates from 1939.[4] The prefix co- means jointly, associated in action, partnered to a lesser degree, etc.; thus a "co-valent bond", in essence, means that the atoms share "valence", such as is discussed in valence bond theory. In the molecule H2, the hydrogen atoms share the two electrons via covalent bonding. Covalency is greatest between atoms of similar electronegativities. Thus, covalent bonding does not necessarily require the two atoms be of the same elements, only that they be of comparable electronegativity. Although covalent bonding entails sharing of electrons, it is not necessarily delocalized. Furthermore, in contrast to electrostatic interactions ("ionic bonds") the strength of covalent bond depends on the angular relation between atoms in polyatomic molecules.

Contents

History

Early concepts in covalent bonding arose from this kind of image of the molecule of methane. Covalent bonding is implied in the Lewis structure that indicates sharing of electrons between atoms.

The term "covalence" in regard to bonding was first used in 1919 by Irving Langmuir in a Journal of the American Chemical Society article entitled "The Arrangement of Electrons in Atoms and Molecules". Langmuir wrote that "we shall denote by the term covalence the number of pairs of electrons which a given atom shares with its neighbors."[5]

The idea of covalent bonding can be traced several years before 1919 to Gilbert N. Lewis, who in 1916 described the sharing of electron pairs between atoms.[6] He introduced the Lewis notation or electron dot notation or Lewis dot structure in which valence electrons (those in the outer shell) are represented as dots around the atomic symbols. Pairs of electrons located between atoms represent covalent bonds. Multiple pairs represent multiple bonds, such as double and triple bonds. Some examples of Electron Dot Notation are shown in the following figure. An alternative form of representation, not shown here, has bond-forming electron pairs represented as solid lines. While the idea of shared electron pairs provides an effective qualitative picture of covalent bonding, quantum mechanics is needed to understand the nature of these bonds and predict the structures and properties of simple molecules. Walter Heitler and Fritz London are credited with the first successful quantum mechanical explanation of a chemical bond, specifically that of molecular hydrogen, in 1927.[7] Their work was based on the valence bond model, which assumes that a chemical bond is formed when there is good overlap between the atomic orbitals of participating atoms. These atomic orbitals are known to have specific angular relationships between each other, and thus the valence bond model can successfully predict the bond angles observed in simple molecules.

Physical properties of covalent compounds (polar/non-polar)

Physical properties Covalent compounds
States (at room temperature) Solid, liquid, gas
Electrical conductivity Usually none
Boiling point and Melting point Varies, but usually lower than ionic compounds
Solubility in water Varies, but usually lower than ionic compounds
Thermal conductivity Usually low

Polarity of covalent bonds

Covalent bonds are affected by the electronegativity of the connected atoms. Two atoms with equal electronegativity will make non-polar covalent bonds such as H-H. An unequal relationship creates a polar covalent bond such as with H-Cl.

Subdivision of covalent bonds

There are three types of covalent substances: individual molecules, molecular structures, and macromolecular structures. Individual molecules have strong bonds that hold the atoms together, but there are negligible forces of attraction between molecules. Such covalent substances are gases. For example, HCl, SO2, CO2, and CH4. In molecular structures, there are weak forces of attraction. Such covalent substances are low-boiling-temperature liquids (such as ethanol), and low-melting-temperature solids (such as iodine and solid CO2). Macromolecular structures have large numbers of atoms linked in chains or sheets (such as graphite), or in 3-dimensional structures (such as diamond and quartz). These substances have high melting and boiling points, are frequently brittle, and tend to have high electrical resistivity. Elements that have high electronegativity, and the ability to form three or four electron pair bonds, often form such large macromolecular structures.[8]

See also

References

  1. ^ Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 0-13-250882-6. http://www.phschool.com/el_marketing.html. 
  2. ^ March, J. “Advanced Organic Chemistry” 4th Ed. J. Wiley and Sons, 1991: New York. ISBN 0-471-60180-2.
  3. ^ G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6.
  4. ^ Merriam-Webster – Collegiate Dictionary (2000).
  5. ^ Langmuir, Irving (1919-06-01). "The Arrangement of Electrons in Atoms and Molecules". Journal of the American Chemical Society 41 (6): 868–934. doi:10.1021/ja02227a002. 
  6. ^ Lewis, Gilbert N. (1916-04-01). "The atom and the molecule". Journal of the American Chemical Society 38 (4): 762–785. doi:10.1021/ja02261a002. 
  7. ^ W. Heitler and F. London, Zeitschrift für Physik, vol. 44, p. 455 (1927). English translation in Hinne Hettema (2000). Quantum chemistry: classic scientific papers. World Scientific. pp. 140–. ISBN 978-981-02-2771-5. http://books.google.com/books?id=qsidHRJmUoIC&pg=140. Retrieved 3 November 2011. 
  8. ^ Stranks, D. R.; M. L. Heffernan, K. C. Lee Dow, P. T. McTigue, G. R. A. Withers (1970). Chemistry: A structural view. Carlton, Victoria: Melbourne University Press. p. 184. ISBN 0 522 83988 6. 

Sources

External links


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Look at other dictionaries:

  • covalent bond — Bond Bond (b[o^]nd), n. [The same word as band. Cf. {Band}, {Bend}.] 1. That which binds, ties, fastens, or confines, or by which anything is fastened or bound, as a cord, chain, etc.; a band; a ligament; a shackle or a manacle. [1913 Webster]… …   The Collaborative International Dictionary of English

  • covalent bond — n a chemical bond formed between atoms by the sharing of electrons * * * a chemical bond between two atoms or radicals formed by the sharing of a pair (single bond), two pairs (double bond), or three pairs of electrons (triple bond) …   Medical dictionary

  • covalent bond — n. the chemical bond formed between two atoms when they share electrons in pairs, so that each atom provides half the electrons: see IONIC BOND, COORDINATE BOND …   English World dictionary

  • covalent bond — Chem. the bond formed by the sharing of a pair of electrons by two atoms. [1960 65] * * * Force holding atoms in a molecule together as a specific, separate entity (as opposed to, e.g., colloidal aggregates; see bonding). In covalent bonds, two… …   Universalium

  • covalent bond — noun a chemical bond that involves sharing a pair of electrons between atoms in a molecule • Hypernyms: ↑chemical bond, ↑bond • Hyponyms: ↑double bond, ↑coordinate bond, ↑dative bond * * * noun : a nonionic chemical bond formed …   Useful english dictionary

  • covalent bond — UK [kəʊˌveɪlənt ˈbɒnd] / US [koʊˌveɪlənt ˈbɑnd] noun [countable] Word forms covalent bond : singular covalent bond plural covalent bonds chemistry a chemical bond between two atoms produced when electrons are shared …   English dictionary

  • covalent bond — /koʊveɪlənt ˈbɒnd/ (say kohvayluhnt bond) noun a chemical bond formed by the sharing of electrons between two atoms each of which donates an equal number of electrons. The conventional single covalent bond involves the sharing of two electrons,… …   Australian English dictionary

  • covalent bond — kovalentinis ryšys statusas T sritis chemija apibrėžtis Ryšys, atsirandantis dėl kelių atomų valentinių elektronų orbitalių sanklotos. atitikmenys: angl. covalent bond; valence bond rus. атомная связь; ковалентная связь ryšiai: sinonimas –… …   Chemijos terminų aiškinamasis žodynas

  • covalent bond — kovalentinis ryšys statusas T sritis fizika atitikmenys: angl. covalent bond vok. kovalente Bindung, f rus. ковалентная связь, f pranc. liaison covalente, f …   Fizikos terminų žodynas

  • covalent bond — cova′lent bond′ n. chem. the bond formed by the sharing of a pair of electrons by two atoms • Etymology: 1960–65 …   From formal English to slang

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