A covalent bond is a chemical bond formed when two atoms share two valence electrons, one contributed by each of the atoms. Unlike ionic bonds, covalent bonds connect specific atoms and so can give rise to true, discrete molecules. Substances with covalent bonds – mostly organic compounds – tend to have low melting and boiling points and to be soluble in nonpolar solvents.
The electrons in a covalent bond are shared equally only when the atoms are identical. In most covalent bonds the electrons are held to a greater extent by one atom than by the other. See also electronegativity.
The covalent bond is the classical electron pair or homopolar bond of chemistry, particularly of organic chemistry. It is a strong bond: the bond between two carbon atoms in diamond, for example, has a cohesive energy of 7.3 eV with respect to separated neutral atoms. This is comparable with the bond strength in ionic crystals, in spite of the fact that the covalent bond acts between neutral atoms. The covalent bond has strong directional properties. Thus carbon, silicon, and germanium have the diamond structure, with atoms joined to four nearest neighbors at tetrahedral angles, even though this arrangement gives a low filling of space.
The covalent bond is usually formed from two electrons, one from each atom participating in the bond. The electrons forming the bond tend to be partly localized in the region between the two atoms joined by the bond. The spins of the two electrons in the bond are antiparallel.