A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds. A chemical bond is the physical process responsible for the attractive interactions between Atoms and Molecules and which confers stability to diatomic and polyatomic The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny In short, attraction-to-repulsion stability that forms between atoms when they share electrons is known as covalent bonding.

Covalent bonding includes many kinds of interactions, including σ-bonding, π-bonding, metal-metal bonding, agostic interactions, and three-center two-electron bonds. In Chemistry, sigma bonds ( σ bonds) are the strongest type of covalent Chemical bond. In Chemistry, pi bonds ( π bonds) are covalent Chemical bonds where two lobes of one involved electron orbital overlap two lobes Agostic interaction is a term in Organometallic chemistry, for the interaction of a coordinately-unsaturated Transition metal with a C-H bond, when the A three-center two-electron bond is an electron deficient Chemical bond where three atoms share two electrons ^[1][2] The term covalent bond dates from 1939. ^[3] The prefix co- means jointly, associated in action, partnered to a lesser degree, etc. ; thus a "co-valent bond", essentially, means that the atoms share "valence", such as is discussed in valence bond theory. In Chemistry, valence, also known as valency or valency number, is a measure of the number of Chemical bonds formed by the Atoms In Chemistry, valence bond theory explains the nature of a Chemical bond in a Molecule in terms of atomic valencies. In the molecule H₂, the hydrogen atoms share the two electrons via covalent bonding. Covalency is greatest between atoms of similar electronegativities. " Electronegativity " is the opposite of " Electropositivity," which describes an element's ability to donate electrons Thus, covalent bonding does not necessarily require the two atoms be of the same elements, only that they be of comparable electronegativity. Because covalent bonding entails sharing of electrons, it is necessarily delocalized. In chemistry delocalized electrons are Electrons in a Molecule that are not associated with a single Atom or to a Covalent bond. Furthermore, in contrast to electrostatic interactions ("ionic bonds") the strength of covalent bond depends on the angular relation between atoms in polyatomic molecules. An ionic bond (or electrovalent bond) is a type of Chemical bond that can often form between Metal and Non-metal Ions (or

MO diagrams depicting covalent (left) and polar covalent (right) bonding in a diatomic molecule. A molecular orbital diagram or MO diagram for short is a qualitative descriptive tool explaining Chemical bonding in Molecules in terms of Molecular The arrows represent electrons provided by the participating atoms.

History

The term "covalence" in regard to bonding was first used in 1919 by Irving Langmuir in a Journal of American Chemical Society article entitled The Arrangement of Electrons in Atoms and Molecules:^[4]

The idea of covalent bonding can be traced several years prior to 1920 to Gilbert N. Lewis, who in 1916 described the sharing of electron pairs between atoms. Gilbert Newton Lewis ( October 23, 1875 - March 23, 1946) was a famous American physical chemist known for the discovery He introduced the so called Lewis notation or electron dot notation or The Lewis Dot Structure in which valence electrons (those in the outer shell) are represented as dots around the atomic symbols. Lewis structures, also called Lewis-dot diagrams are diagrams that show the bonding between Atoms of a 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, in which bond-forming electron pairs are represented as solid lines, is shown alongside.

Early concepts in covalent bonding arose from this kind of image of the molecule of methane. Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. Covalent bonding is implied in the Lewis structure that indicates sharing of electrons between atoms. Lewis structures, also called Lewis-dot diagrams are diagrams that show the bonding between Atoms of a

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. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons 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. Walter Heinrich Heitler ( 2 January 1904 &ndash 15 November 1981) was a German physicist who made contributions to Quantum electrodynamics Fritz Wolfgang London ( March 7, 1900 &ndash March 30, 1954) was a German -born American theoretical Physicist. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 ^[5] 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. An atomic orbital is a Mathematical function that describes the wave-like behavior of an electron in an atom 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.

Bond order

Bond order is a number that indicates the number of pairs of electrons shared between atoms forming a covalent bond. Bond order is the number of bonds between a pair of atoms For example in Nitrogen N≡N the bond order is 3 in Acetylene H−C≡C−H the bond order between the The term is only applicable to diatomic molecules, but is used to describe bonds within polyatomic compounds as well. Diatomic molecules are molecules made only of two Atoms of either the same or different Chemical elements The prefix di- means two in Greek

1. The most common type of covalent bond is the single bond, the sharing of only one pair of electrons between two atoms. It usually consists of one sigma bond. In Chemistry, sigma bonds ( σ bonds) are the strongest type of covalent Chemical bond. All bonds with more than one shared pair are called multiple bonds.
2. Sharing two pairs is called a double bond. An example is in ethylene (between the carbon atoms). Structure This Hydrocarbon has four Hydrogen Atoms bound to a pair of Carbon atoms that are connected by a Double bond. It usually consists of one sigma bond and one pi bond. In Chemistry, sigma bonds ( σ bonds) are the strongest type of covalent Chemical bond. In Chemistry, pi bonds ( π bonds) are covalent Chemical bonds where two lobes of one involved electron orbital overlap two lobes
3. Sharing three pairs is called a triple bond. An example is in hydrogen cyanide (between C and N). Hydrogen cyanide is a Chemical compound with Chemical formula HCN It usually consists of one sigma bond and two pi-bonds.
4. Quadruple bonds are found in the transition metals. A quadruple bond is a type of Chemical bond between two Atoms involving 8 Electrons This bond is an extension of the more familiar types Double bonds Molybdenum and rhenium are the elements most commonly observed with this bonding configuration. Molybdenum (məˈlɪbdənəm from the Greek word for the metal " Lead " is a Group 6 Chemical element with the symbol Mo Rhenium (ˈriːniəm is a Chemical element with the symbol Re and Atomic number 75 An example of a quadruple bond is also found in Di-tungsten tetra(hpp). Di-tungsten tetra(hpp or W2(hpp4 is a di- Tungsten paddlewheel compound with four hpp or hexahydropyrimidopyrimidine
5. Quintuple bonds have been found to exist in certain dichromium compounds. A quintuple bond in Chemistry is an unusual type of Chemical bond first reported in 2005 for a di Chromium compound Chromium (ˈkroʊmiəm is a Chemical element which has the symbol Cr and Atomic number 24
6. Sextuple bonds are found in diatomic molybdenum and tungsten. A sextuple bond is a type of Covalent bond involving 12 bonding Electrons The only known molecules with true sextuple bonds (order 6 are diatomic Mo 2 Molybdenum (məˈlɪbdənəm from the Greek word for the metal " Lead " is a Group 6 Chemical element with the symbol Mo Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74

Most bonding of course, is not localized, so the above classification, while powerful and pervasive, is of limited validity. Three-center bonds do not conform readily to the above conventions. A three-center two-electron bond is an electron deficient Chemical bond where three atoms share two electrons

Resonance

Many bonding situations can be described with more than one valid Lewis Dot Structure (for example, ozone, O₃). OZONE is an object oriented Operating system written in the C programming language. In an LDS diagram of O₃, the center atom will have a single bond with one atom and a double bond with the other. The LDS diagram cannot tell us which atom has the double bond; the first and second adjoining atoms have equal chances of having the double bond. These two possible structures are called resonance structures. Resonance in Chemistry is a theory used to represent and model certain types of non-classical Molecular structures Resonance is a key component In reality, the structure of ozone is a resonance hybrid between its two possible resonance structures. Instead of having one double bond and one single bond, there are actually two 1. 5 bonds with approximately three electrons in each at all times.

A special resonance case is exhibited in aromatic rings of atoms (for example, benzene). Benzene, or benzol, is an organic Chemical compound and a known Carcinogen with the molecular formula C 6 H 6 Aromatic rings are composed of atoms arranged in a circle (held together by covalent bonds) that may alternate between single and double bonds according to their LDS. In actuality, the electrons tend to be disambiguously and evenly spaced within the ring. Electron sharing in aromatic structures is often represented with a ring inside the circle of atoms.

Lewis Dot Structures for molecules with resonance are shown by creating the dot structure for every possible form, placing brackets around each structure,and connecting the boxes with double-headed arrows.

Current theory

Today the valence bond model has been supplanted by the molecular orbital model. In Chemistry, a molecular orbital (or MO) is a region in which an Electron may be found in a Molecule. In this model, as atoms are brought together, the atomic orbitals interact to form molecular orbitals, which are linear sums and differences of the atomic orbitals. These molecular orbitals are a cross between the original atomic orbitals and generally extend between the two bonding atoms.

Using quantum mechanics it is possible to calculate the electronic structure, energy levels, bond angles, bond distances, dipole moments, and electromagnetic spectra of simple molecules with a high degree of accuracy. Bond distances and angles can be calculated as accurately as they can be measured (distances to a few pm and bond angles to a few degrees). For small molecules, calculations are sufficiently accurate to be useful for determining thermodynamic heats of formation and kinetic activation energy barriers.

See also

• Metallic bonding
• Linear combination of atomic orbitals
• Hybridization
• Hydrogen bond
• Noncovalent bonding
• Disulfide bond

References

1. ^ March, J. Although the term metallic bond is often used in contrast to the term Covalent bond it is better to speak of metallic bonding, because this type of bonding is A linear combination of atomic orbitals or LCAO is a Quantum superposition of Atomic orbitals and a technique for calculating Molecular orbitals -->In Chemistry A hydrogen bond results from a Dipole-dipole force between an Electronegative atom and a Hydrogen atom bonded to Nitrogen, Oxygen A noncovalent bond is a type of Chemical bond, typically between Macromolecules that does not involve the sharing of pairs of electrons but rather involves more dispersed In Chemistry, a disulfide bond is a single Covalent bond derived from the coupling of Thiol groups “Advanced Organic Chemistry” 4th Ed. J. Wiley and Sons, 1991: New York. ISBN 0-471-60180-2.
2. ^ G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/PrentButt holes suckice Hall publisher, ISBN 0-13-035471-6.
3. ^ Merriam-Webster - Collegiate Dictionary (2000). Merriam-Webster, which was originally the G & C Merriam Company of Springfield Massachusetts, is an American company that publishes reference books
4. ^ Langmuir, I. (1919). J. Am. Chem. Soc. ; 1919; 41; 868-934.
5. ^ W. Heitler and F. London, Zeitschrift für Physik, vol. 44, p. 455 (1927). English translation in H. Hettema, Quantum Chemistry, Classic Scientific Papers, World Scientific, Singapore (2000).

• Covalent bonding - Single bonds. chemguide (2000).
• Electron Sharing and Covalent Bonds. Department of Chemistry University of Oxford.
• Chemical Bonds. Department of Physics and Astronomy, Georgia State University.

External links

• Covalent Bonds and Molecular Structure