Covalent bonds are much more common in organic chemistry than ionic bonds. A covalent bond consists of the simultaneous attraction of two nuclei for one or more pairs of electrons. The electrons located between the two nuclei are bonding electrons. Covalent bonds occur between identical atoms or between different atoms whose difference in electronegativity is insufficient to allow transfer of electrons to form ions.

A single covalent bond in which both electrons in the shared pair come from the same atom is called a coordinate covalent bond. To indicate a coordinate covalent bond an arrow is sometimes drawn from the atom that donates the electron pair toward the atom with which the pair is shared. The donor atom provides both electrons to a coordinate covalent bond and the acceptor atom accepts an electron pair for sharing in a coordinate covalent bond. For coordinate covalent bonds, as for any other kind of bond, it is impossible to distinguish among the electrons once the bond has formed. For example, a hydrogen ion unites with an ammonia molecule by a coordinate covalent bond to form the ammonium ion The two hydrogen nuclei are separated by a distance called the bond length. This distance results from a balance between attractive and repulsive forces. There is an attraction between the nuclei and the bonding electrons, but there is also a repulsion between the two nuclei as well as between the two electrons. a schematic diagram of these attractive and repulsive forces. It provides a starting point for our discussion of bonding. When a covalent bond forms between two hydrogen atoms, there are two sets of electrostatic repulsions (nuclear–nuclear and electron–electron, red), but four sets of electrostatic attractions (green). The attractive forces are equal in magnitude, but opposite in sign. Each hydrogen nucleus attracts both electrons. The net result is that the energy of the system decreases when the bond forms. This simple electrostatic model for bonding does not adequately describe chemical bonds.