The sigma bond (represented as σ) is a covalent bond, which is characterized by the sharing of two electrons that occurs between a pair of atoms to form a said bond. In addition, this is a kind of single bond, in which both atoms are attached by two electrons to form a single bond.
When two or more atoms combine to give rise to new molecular compounds, they are joined by two types of bonds: ionic and covalent, whose structure depends on how the electrons are shared between the two atoms involved in this coupling.
The connection generated through the electrons is carried out thanks to the overlapping of the orbitals that belong to each atom (by their ends), understanding as orbitals the spaces where it is most likely to locate the electron in the atom and that are defined by the density electronics.
How is a sigma bond formed?
Typically, the single bond between two atoms is known to be equivalent to a single sigma-like bond.
These atoms whose orbitals overlap must be adjacent to each other so that the individual electrons belonging to each atomic orbital can effectively bond and form the bond.
Hence the fact that the electronic distribution that manifests itself or the location of the density of the electrons from each superposition has a cylindrical symmetry around the axis that occurs between the two linked atomic species.
In this case, the so-called sigma orbital can be more easily expressed in terms of intramolecular bonds that form within diatomic molecules, noting that there are also several types of sigma bonds.
The most commonly observed types of sigma bonds are: d z 2 + d z 2, s + p z, p z + p z, and s + s; where the subscript z represents the axis constituted by the bond formed and each letter (s, p, and d) corresponds to an orbital.
When we speak of molecular orbitals, reference is made to the regions that accumulate the highest electron density when a bond of this type is formed between different molecules, which is obtained through the combination of atomic orbitals.
From the point of view of quantum mechanics, studies have inferred that molecular-type orbitals that exhibit symmetrically equal behavior are actually combined in mixtures (hybridizations).
However, the significance of this combination of orbitals is closely related to the relative energies manifested by molecular-type orbitals that are symmetrically similar.
In the case of organic molecules, cyclic species consisting of one or more ring structures are frequently observed, which are frequently constituted by a large number of sigma-type bonds in conjunction with pi-type bonds (multiple bonds).
In fact, using simple mathematical calculations, it is possible to determine the number of sigma bonds present in a molecular species.
There are also cases of coordination compounds (with transition metals), in which multiple bonds are combined with different classes of binding interactions, as well as molecules made up of different types of atoms (polyatomic).
Sigma bonds have unique characteristics that clearly differentiate them from other types of covalent bonding (pi bond), among which is the fact that this type of bond is the strongest among the covalent class chemical bonds.
This is because the overlap between the orbitals occurs directly, coaxially (or linearly), and frontally; that is, a maximum overlap between the orbitals is obtained.
Additionally, the electronic distribution at these junctions is mainly concentrated between the nuclei of the atomic species that are combined.
This overlap of sigma orbitals occurs in three possible ways: between a pair of pure orbitals (ss), between a pure orbital and a hybrid type (s-sp) or between a pair of hybrid type orbitals (sp 3 – sp 3 ).
Hybridization occurs thanks to the mixture of orbitals of atomic origin of different classes, obtaining that the resulting hybrid orbital depends on the amount of each of the starting pure orbital types (for example, sp 3 = a pure s + orbital three pure p-type orbitals).
In addition to this, the sigma bond can exist independently, as well as admit free rotational movement between a pair of atoms.
Examples of a sigma bond
Since the covalent bond is the most common kind of bond between atoms, the sigma bond is found in a huge number of chemical species, as can be seen below.
In diatomic gas molecules – such as hydrogen (H 2 ), oxygen (O 2 ), and nitrogen (N 2 ) – different types of bonds can occur depending on the hybridization of the atoms.
In the case of hydrogen, there is a single sigma bond joining both atoms (H – H), because each atom contributes its only electron.
On the other hand, in molecular oxygen both atoms are linked by a double bond (O = O) —that is, a sigma bond — and a pi bond, leaving each atom with three pairs of remaining electrons paired.
Instead, each nitrogen atom has five electrons in its outermost energy level (valence shell), so they are joined by a triple bond (N≡N), which implies the presence of a sigma bond and two pi bonds and a pair of paired electrons in each atom.
In the same way, it occurs in cyclic-type compounds with single or multiple bonds and in all kinds of molecules whose structure is made up of covalent bonds.
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