In chemistry and allied sciences, a chemical bond is the force which holds atoms together in molecules or crystals. In simple compounds, concepts such as valency or oxidation number can help determine whether or not atoms can bond with other atoms of the same or different type in a certain way. With more complicated compounds, such as complexes, valence concepts fail and a more thorough understanding of quantum mechanics is necessary.
The spatial characteristics and range of energies encompassed by chemical forces span a continuum, making the terms for the different types of chemical bond somewhat arbitrary or overlapping in their applicability, but the types include:
electronic configuration of the molecule or substance (in the case of extended array structures, such as found in crystals). The types of bonding are differentiated by the extent to which electron density is localized or delocalized among the atoms of the substance. In the case of ionic bonding, electrons are more tightly associated with individual atoms, with net charges being assigned to discrete constituent atoms throughout the substance, with the nature of the interatomic (or more appropriately) interionic forces largely characterized by isotropic continuum electrostatic potentials.
In contrast, the electron density distributions within covalent bonds are not so readily assigned to individual atoms, but are instead delocalized across the molecule in structures sometimes described as molecular orbitals, which may have more directed, anisotropic properties. Intermediate situations certainly exist, with bonds having some mix of polarized ionic-like nature and some more electronically dispersed structure.