共价键(covalent bond)
化学术语，指由於两个原子（在少数情况下为三个原子）的原子核对相同电子的静电吸引而在它们之间形成的化学键。每个共价键包括两个自旋成对（自旋相反）的电子，因此有时也称作电子对键。在分子结构式中，共价键以连接两个原子的实线表示：单线表示两个原子之间的单键，涉及一个电子对；双线表示双键，涉及两个电子对；三线表示三键，涉及三个电子对。两个电子可为两个原子核同时吸引，亦即两个电子为两个原子所共享，而且起着连接这些原子的作用。这一概念是美国化学家路易斯（G. N. Lewis）於1916年首次提出。他指出这种键的形成是由於某些原子具有彼此成对，从而使两者都具有相应的稀有气体原子的电子构型的倾向。1927年物理学家海脱勒（W. Heitler）、伦敦（F. London）等科学家证实了路易斯的概念，并根据量子力学方程式计算氢分子键性质（即键能和键长）的准确值。参阅ionic bond。
English version：
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, one or more pairs of valence electrons are shared between two atoms to give each the stability found in a noble gas. In single bonds (e.g., H&singlehorzbond;H in H2), one electron pair is shared; in double bonds (e.g., O&doublehorzbond;O in O2 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 group, like sulfate or phosphate. 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 has four single bonds, each pointing to one vertex of a tetrahedron; making certain molecules exist in mirror images (see optical activity). Double bonds are rigid, leading to the possibility of geometric isomers (see isomerism). Some types of bonds, like the amide linkages that join the amino acids in peptides and proteins (peptide bonds), though apparently single, 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 ionic bond.