Covalent Bond (redirected from Polar bonds)

covalent bond (kō'vā`lənt): see chemical bond.

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covalent bond

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., H−H in molecular hydrogen), one electron pair is shared; in double bonds (e.g., O=O in molecular oxygen or H₂C=CH₂ in ethylene), two; in triple bonds (e.g., HC≡CH 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. See also aromatic compound; compare ionic bond.

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covalent bond [kō′vā·lənt ′bänd]

(chemistry)

A bond in which each atom of a bound pair contributes one electron to form a pair of electrons. Also known as electron pair bond.

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Covalent Bond 

a chemical bond between two atoms that is formed by a shared electron pair (one electron from each atom). Covalent bonds exist both in molecules (in all states of aggregation) and between the atoms that form crystal lattices. They may unite like atoms (in H₂ and C₁₂ molecules and diamond crystals) or unlike atoms (in water molecules and crystals of carborundum, SiC).

Almost all the main bonds in the molecules of organic compounds are covalent (C—C, C—H, C—N, and so on). Covalent bonds are very strong, which explains the low chemical activity of paraffin hydrocarbons. Many inorganic compounds whose crystals have an atomic lattice (that is, are formed by a covalent bond) are refractory and characterized by great hardness and wear resistance; among them are certain carbides, suicides, borides, and nitrides (particularly borazone, BN), which are widely used in modern technology.

V. A. KIREEV