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:
In all types of
bonding, the 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.
Ionic bonding can largely be
described by classical mechanics, but the complexity of
covalent bonding relies more heavily on concepts from quantum mechanics.
Other types of interatomic and
intermolecular forces that occur within the chemical energy regime, but which
are not characterized as bonds are:
See also: atomic orbitals, double bond, periodic
table