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Covalent bonding 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. 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 complexs, and three-center two-electron bonds.March, J. “Advanced Organic Chemistry” 4th Ed. J. Wiley and Sons, 1992: New York. ISBN 0-471-60180-2.G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6. The term
covalent bond dates from 1939.Merriam-Webster - Collegiate Dictionary (2000). 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 (chemistry)", 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
electronegativity. 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 electron. Furthermore, in contrast to electrostatic interactions ("ionic bonds") the strength of covalent bond depends on the angular relation between atoms in polyatomic molecules.
History
The term "covalence" in regards 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:Langmuir, I. (1919).
J. Am. Chem. Soc.; 1919; 41; 868-934.
The idea of covalent bonding can be traced several years prior to 1919 to
Gilbert N. Lewis, who in 1916 described the sharing of electron pairs between atoms. He introduced the so called
Lewis Structure 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. 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.. Covalent bonding is implied in the
Lewis structure that indicates sharing of electrons between atoms.
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.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). 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.
== Bond order ==
Bond order is a number that indicates the number of pairs of electrons shared between atoms forming a covalent bond. The term is only applicable to diatomic molecules, but is used to describe bonds within polyatomic compounds as well.
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. All bonds with more than one shared pair are called multiple bonds.
Sharing two pairs is called a double bond. An example is in ethylene (between the carbon atoms). It usually consists of one sigma bond and one pi bond.
Sharing three pairs is called a triple bond. An example is in hydrogen cyanide (between C and N). It usually consists of one sigma bond and two pi-bonds.
Quadruple bonds are found in the transition metals. Molybdenum and rhenium are the elements most commonly observed with this bonding configuration. An example of a quadruple bond is also found in Di-tungsten tetra(hpp).
Quintuple bonds have been found to exist in certain dichromium compounds.
The only known molecules with true sextuple bonds (order 6) are diatomic molybdenum2 and tungsten2, in the gaseous phase at very low temperatures. Although diatomic chromium2 and uranium2 have formal structures with twelve-electron bonds, their effective bond orders (derived from quantum chemistry calculations) are less than 5. There is strong evidence to believe that no two elements in the periodic table can form a bond with greater order than 6.
Most bonding of course, is not localized, so the following classification, while powerful and pervasive, is of limited validity. Three center bond do not conform readily to the above conventions.
Resonance
Many bonding situations can be described with more than one valid Lewis Dot Structure (for example,
ozone, O3). 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
chemical resonance. 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). 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.
Current theory
Today the valence bond model has been supplanted by the molecular orbital model. 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
References
External links
- Covalent Bonds and Molecular Structure
Covalent bonding 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. 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 complexs, and three-center two-electron bonds.March, J. “Advanced Organic Chemistry” 4th Ed. J. Wiley and Sons, 1992: New York. ISBN 0-471-60180-2.G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6. The term
covalent bond dates from 1939.Merriam-Webster - Collegiate Dictionary (2000). 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 (chemistry)", 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 electronegativity. 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 electron. Furthermore, in contrast to electrostatic interactions ("
ionic bonds") the strength of covalent bond depends on the angular relation between atoms in polyatomic molecules.
History
The term "covalence" in regards 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:Langmuir, I. (1919).
J. Am. Chem. Soc.; 1919; 41; 868-934.
The idea of covalent bonding can be traced several years prior to 1919 to Gilbert N. Lewis, who in 1916 described the sharing of electron pairs between atoms. He introduced the so called
Lewis Structure 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. 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.. Covalent bonding is implied in the
Lewis structure that indicates sharing of electrons between atoms.
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.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). 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.
== Bond order ==
Bond order is a number that indicates the number of pairs of electrons shared between atoms forming a covalent bond. The term is only applicable to diatomic molecules, but is used to describe bonds within polyatomic compounds as well.
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. All bonds with more than one shared pair are called multiple bonds.
Sharing two pairs is called a double bond. An example is in ethylene (between the carbon atoms). It usually consists of one sigma bond and one pi bond.
Sharing three pairs is called a triple bond. An example is in hydrogen cyanide (between C and N). It usually consists of one sigma bond and two pi-bonds.
Quadruple bonds are found in the transition metals. Molybdenum and rhenium are the elements most commonly observed with this bonding configuration. An example of a quadruple bond is also found in Di-tungsten tetra(hpp).
Quintuple bonds have been found to exist in certain dichromium compounds.
The only known molecules with true sextuple bonds (order 6) are diatomic molybdenum2 and tungsten2, in the gaseous phase at very low temperatures. Although diatomic chromium2 and uranium2 have formal structures with twelve-electron bonds, their effective bond orders (derived from quantum chemistry calculations) are less than 5. There is strong evidence to believe that no two elements in the periodic table can form a bond with greater order than 6.
Most bonding of course, is not localized, so the following classification, while powerful and pervasive, is of limited validity.
Three center bond do not conform readily to the above conventions.
Resonance
Many bonding situations can be described with more than one valid Lewis Dot Structure (for example, ozone, O3). 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
chemical resonance. 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). 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.
Current theory
Today the valence bond model has been supplanted by the
molecular orbital model. 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
- Chemical bond
- Metallic bonding
- Ionic bond
- Linear combination of atomic orbitals
- Orbital hybridisation
- Hydrogen bond
- Noncovalent bonding
- Disulfide bond
References
External links
- Covalent Bonds and Molecular Structure
Structure and Bonding: Covalent Bonds
In the previous page, we saw how atoms could achieve a complete shell of electrons by losing or gaining one or more electrons, to form ions. There is another way atoms can satisfy ...
covalent bonding - single bonds
Explains how single covalent bonds are formed, starting with a simple view and then extending it for A'level.
covalent bonding - double bonds
Explains how double covalent bonds are formed, starting with a simple view and then extending it for A'level.
Covalent bond - Wikipedia, the free encyclopedia
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.
Covalent Bond
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Covalent bonds can be represented in several different ways. ... Representing covalent bonds. Covalent bonds can be represented in several different ways.
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Encyclopedia article about covalent bond. Information about covalent bond in the Columbia Encyclopedia, Computer Desktop Encyclopedia, computing dictionary. covalent bonds
