In chemistry, a chemical bond is the force, which holds together atoms in molecules or crystals. In many simple compounds, valence bond theory and the concept of oxidation number can be used to predict molecular structure and composition. Similarly, theories from classical physics can be used to predict many ionic structures. With more complicated compounds, such as metal complexes, valence bond theory fails and a more thorough understanding based on quantum mechanics is necessary.
The spatial characteristics and range of energies encompassed by chemical forces span a continuum, so the terms for the different types of chemical bond overlap in their applicability, but the types include
All chemical bonding arises from the energetically favourable (that is, low-energy) interaction between electrons on different atoms. The types of bonding are distinguished 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 mainly associated with individual atoms, and an overall electric charge is assigned to discrete constituent atoms throughout the substance. The nature of the interatomic (or in fact interionic) forces is largely characterized by isotropic continuum electrostatic potentials.
By contrast, in covalent bonding the electron density distributions within bonds are not assigned to individual atoms, but are instead delocalized across the molecule in structures which are described by the most common contemporary theory as molecular orbitals. Unlike pure ionic bonds, these may have directed anisotropic properties. Intermediate situations certainly exist, in which bonds show a mixture between polarized ionic character and electron-delocalized covalent character.
Ionic bonding can largely be described by classical physics, but the complexity of covalent bonding relies more heavily on concepts from quantum mechanics.
Aside from the intramolecular bonds which hold molecules together, intermolecular forces also act to provide an attraction between the molecules of a substance.
Linus Pauling's book The Nature of the Chemical Bond is by some accounts the most influential book on chemistry ever published.