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In chemistry, delocalized electrons are electrons or pairs of electrons belonging to an atom, molecule, or ion that are not confined to revolving around a single chemically bonded atom or pair of atoms, but have some freedom of movement through a molecule or a solid. In other words, the term refers to electrons that are not located in a particular atom or covalent bond.
The delocalized electrons can be either bonding electrons or nonbonding electrons. They can also be present in both atomic orbitals and molecular orbitals. The key to the mobility of electrons that gives rise to delocalization is the combination of different similar orbitals between adjacent atoms. This can occur from the lateral overlap of p orbitals during pi bond formation in double and triple covalent bonds , or it can occur from the combination of the atomic orbitals of the metal atoms in the metal bond.
Delocalized electrons in the covalent bond
According to the valence bond theory, the covalent bond is formed by the overlapping of the atomic orbitals of the valence electrons of the bonded atoms. When two atoms are covalently bonded to each other by sharing more than one pair of electrons, the first pair of electrons forms the sigma bond by the frontal overlap of two atomic orbitals oriented along the axis that joins both atoms.
However, the second and third pairs of electrons that are shared in double and triple bonds, respectively, do so by lateral overlap of the p and p z atomic orbitals of two adjacent atoms, thus forming pi bonds. These orbitals are located above and below the axis that joins the atoms and not directly on this axis as in the case of the sigma bond.
When there is more than one multiple bond in a row through a chain of atoms (called conjugate bonds), the p orbitals that form part of one of the pi bonds also overlap with the p orbitals that form the next. pi bond, thus forming a single pi bond that encompasses all bonded atoms. The bonding electrons found in these orbitals (called pi electrons) are free to move throughout the entire conjugated bond, so they are said to be delocalized.
Delocalization and resonance
The delocalization of the electrons is clearly evidenced by drawing the different Lewis structures of a chemical compound. On many occasions, the same compound can be represented by more than one Lewis structure. Each of these structures can be converted into the others through the movement of pi electrons or nonbonding pairs of electrons through the structure. This process of transformation from one Lewis structure to another is called resonance, and it is a graphic way of seeing the delocalization of electrons.
In many cases, experimental evidence shows that the actual structure is not any of these individual resonance structures, but rather a combination of all resonance structures in what is called a resonance hybrid. Experimental evidence for the existence of a resonance hybrid is at the same time experimental evidence for the delocalization of pi electrons in a molecule.
Representation of delocalized electrons
When we graphically represent a molecule that has delocalized electrons, we do so through the resonance structure. As mentioned above, this structure is a combination of the individual resonance structures in which all sigma bonds remain unchanged; however, the pi bonds between different atoms are sometimes there and sometimes not, so on average they can be represented as somewhere between a double and a single covalent bond.
The first postulated resonance structure was the structure of benzene proposed by Kekulé. In it, the pi electrons were not located in three pi bonds, but were freely rotating around the molecule.
Delocalized electrons in the metallic bond
Metals make up the largest group of elements on the periodic table. These are characterized by having a high electrical conductivity, which shows that the electrons of the atoms that make up a metal have great freedom of movement; in other words, they are delocalized. In this case, the delocalization of the electrons is due to the characteristics of the metallic bond. There are two theories that explain the metallic bond and its properties: the electron gas theory (also called the electron cloud or electron sea theory) and the band theory.
electronic gas theory
In the electron gas theory, metallic solids are considered as a crystal lattice formed by cations that have lost their valence electrons, which flow freely in the interstices of the crystal lattice as if it were a gas formed by electrons (a gas electronic) that diffuses through a porous medium.
In this theory, each metallic atom loses its valence electron or electrons, so that they are no longer located in a single place in the solid. As a consequence, these electrons are said to be delocalized.
band theory
Band theory is a particular application of molecular orbital theory to metallic bonding. In this theory, the metal is considered as a three-dimensional molecule formed by N atoms bonded together. The metallic bond is explained by means of the overlapping of the atomic orbitals of each one of the atoms that form this metallic macromolecule, thus forming a set of N molecular orbitals.
These molecular orbitals can be bonding, antibonding, and nonbonding. The large number of molecular orbitals that are formed end up giving rise to a band of orbitals with almost continuous energy levels between them.
The further combination of empty pod orbitals also gives rise to bands of empty bonding and antibonding orbitals; in the case of metals, these overlap with the molecular orbitals occupied by valence electrons of the atoms that make up the solid. This overlap allows these valence electrons to be easily promoted to empty orbitals that span the entire solid, thus allowing them to move freely through the solid itself, explaining the conductivity of metals.
Examples of delocalized electrons
Pi electrons of graphite
Graphite is a molecular solid made up of layers of carbon atoms bonded together to form a hexagonal lattice of sp 2 -hybridized atoms . In each of these shells, the p z orbital of each carbon atom overlaps with the p z orbitals of the three neighboring atoms, forming a pi electron system that spans the entire surface of the shell. Layer-on-layer stacking results in an extensive delocalized electron system that gives graphite high conductivity along the plane of the layers.
The opposite is true for the other common allotrope of carbon, diamond. This consists of a three-dimensional network of carbon atoms with sp 3 hybridization in which all the carbon atoms form sigma bonds in which the electrons are perfectly located, which makes diamond one of the best electrical insulators known. .
The 3s electrons of sodium
Sodium is an alkali metal that has a single valence electron in the 3s orbital. Whether we view the bonding between sodium atoms from the point of view of electron gas theory or from the point of view of band theory, the 3s valence electron of each sodium atom has complete freedom of movement along length of the metal, representing an example of delocalized electrons.
The 10 pi electrons of naphthalene
Like benzene and other organic compounds, naphthalene’s pi electrons are delocalized and move freely along the surface of the 10-carbon molecule.
References
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Scientific texts. (2006, May 16). Band Theory . ScientificTexts.com. https://www.textoscientificos.com/quimica/inorganica/enlace-metales/teoria-bandas
