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 atoms share one or more pairs of valence electrons to give each atom the stability found in a noble gas. In single bonds (e.g., HsinglehorzbondH in molecular hydrogen), one electron pair is shared; in double bonds (e.g., OdoublehorzbondO in molecular oxygen or H2CdoublehorzbondCH2 in ethylene), two; in triple bonds (e.g., HCtriplehorzbondCH in acetylene), three. In coordinate covalent bonds, additional electron pairs are shared with another atom, usually forming a functional group, such as sulfate (SO4) or phosphate (PO4). The number of bonds and the atoms participating in each (including any additional paired electrons) give molecules their configuration; the slight negative and positive charges at the opposite ends of a covalent bond are the reason most molecules have some polarity (see electrophile; nucleophile). Carbon in organic compounds can have as many as four single bonds, each pointing to one vertex of a tetrahedron; as a result, certain molecules exist in mirror-image forms (see optical activity). Double bonds are rigid, leading to the possibility of geometric isomers (see isomerism). Some types of bonds, such as the amide linkages that join the amino acids in peptides and proteins (peptide bonds), are apparently single but have some double-bond characteristics because of the electronic structure of the participating atoms. The configurations of enzymes and their substrates, determined by their covalent bonds (particularly the peptide bonds) and hydrogen bonds, are crucial to the reactions they participate in, which are fundamental to all life. Seealso aromatic compound; compare ionic bond.
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Covalent bonding includes many kinds of interaction, including σ-bonding, π-bonding, metal-metal bonding, agostic interactions, and three-center two-electron bonds. The term covalent bond dates from 1939. 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 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.
(p.926)… we shall denote by the term covalence the number of pairs of electrons which a given atom shares with its neighbors.
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. 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. 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. 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.
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 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.
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.
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.