Molecular structures

In most polyatomic molecules, electrons remain mainly in the vicinity of the atoms they link. The electrons between two atoms belong to a localized orbital and thus form a localized bond. As a first approximation, we can therefore consider that the molecule is made up of a juxtaposition of bonds between two atoms. This theory of localized bonds makes it possible to explain the formation of certain polyatomic molecules, and to specify their geometry using the theory of orbital hybridization or the theory of valence shell electron pair repulsion (VSEPR).

However, this assumption of localized bonds is no longer valid when we're dealing with conjugated bonds. In this case, the electrons are delocalized within an electron cloud covering all the bonds concerned (resonance); the bonds have therefore lost their individuality.

On the other hand, since localized bonds are virtually independent of the rest of the molecule, their characteristics are more or less constant: length, energy, dipole moment, force constant, ... These characteristics, which are listed in this article, can be used to predict molecular properties or reaction enthalpies. However, a bond is not perfectly localized, even in molecules without conjugated bonds; the characteristics of the bond therefore depend on the molecule in which it is found and its particular situation within that molecule. As a result, the results obtained are only approximate.