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&singlehorzbond;H in molecular hydrogen), one electron pair is shared; in double bonds (e.g., O&doublehorzbond;O in molecular oxygen or H2C&doublehorzbond;CH2 in ethylene), two; in triple bonds (e.g., HC&triplehorzbond;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. &Seealso; aromatic compound; &compare; ionic bond.
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