A chemical bond is a long-standing attraction between atoms, ions, or molecules that permits chemical bonds to form ideally. They are further categorised as ionic bonds, covalent bonds, coordinate bonds and hydrogen bonds. A coordinated bond exists between two atoms in the molecule. Two electrons form one bond. A hydrogen bond is created by the hydrogen atom of one molecule and the negative atom of another molecule.
Let’s understand these aspects with Sigma and Pi Bond with Example.
Bonds in Chemistry
A covalent bond is a chemical bond that comprises the exchange of electron pairs between atoms. These electron pairs are called shared pairs or bond pairs, and the steady balance of attractive and repulsive forces between atoms, when they share electrons, is called covalent bonding.
Various bond parameters resembling bond length, bond angle, and bond enthalpy rely on how the overlapping of atomic orbital occurs. This overlap happens in two significant ways, giving rise to two primary styles of covalent bonds, i.e., sigma and pi bonds. This article will discuss sigma and pi bonds, their examples, similarities, and differences.
What is a Sigma Bond?
Sigma bonds are the strongest form of the covalent chemical bond. They’re shaped by head-to-head overlapping between atomic orbitals and are sometimes referred to as sigma electrons. A Sigma Bond is recognised as a head-on overlap or axial overlap. These bonds are indicated by the symbol σ (sigma), which is the eighteenth letter of the Greek alphabet.
Typically, all single bonds can be sigma bonds and can be created via multiple combinations of atomic orbitals such as:
- S-S Overlapping
In this form of overlapping, one ‘s’ orbital from every partaking atom undergoes head to head overlapping alongside the internuclear axis. An s orbital ought to be half- stuffed before it overlaps with another.
- S-P Overlapping
In this overlapping type, one half-filled s orbital overlaps with one half-filled p orbitals along the internuclear axis, creating a covalent bond.
- P-P Overlapping
In this condition, one half-filled p orbital from each partaking atom undergoes head to head overlapping along the internuclear axis.
Examples of Sigma Bond
Anything bonded by a single covalent bond is an example of a Sigma bond, such as the bond between two hydrogen atoms. The bonds between the sp3 orbitals of hybridised carbon and the s orbitals of hydrogen in methane are also examples of sigma bonds.
What is a Pi Bond?
Pi bonds are created by positive lateral overlapping (same phase) of atomic orbitals perpendicular to the internuclear axis. Pi bonds are sometimes written as “𝛑 bonds”, with the Greek letter “𝛑” indicating the similar symmetry of the pi bond and the p orbital.
When π bonds are formed, the axes of the atomic orbitals are parallel, while the overlap is perpendicular to the internuclear axis. It is also to be noted that the bonding is only possible between two ‘p’ orbitals.
Two pi bonds are the most that can exist between a given combination of atoms. Quadruple bonds are rare and may be shared only between transition metal atoms and encompass one sigma bond, two pi bonds, and one delta bond. In pi bonds, the electron density is concentrated in the region perpendicular to the bond axis.
Pi bonds are generally weaker than sigma bonds due to the significantly lower degree of overlap. In general, double bonds consist of a sigma bond and a pi bond, while a typical triple bond consists of two π bonds and one σ bond. It is important to note that a combination of sigma and pi bonds is stronger than a single sigma bond.
Examples of Pi bond
Two examples of pi bonds are bonding in Ethene (C2H4), where double bonds consist of a single sigma bond and a single pi bond, or bonding in Ethyne (C2H2).
Similarities between Sigma Bonds and Pi Bonds
Sigma bonds and pi bonds are based on certain molecular orbitals derived from the conjoining of specific atomic orbitals, such as s orbitals for sigma bonds and p orbitals for pi bonds.
They can also be stable or unstable, depending on whether the electrons are in the bonding molecular or the antibonding molecular orbitals.
Difference between Sigma Bonds and Pi Bonds
Sigma Bonds and Pi Bonds have distinctive qualities that make them different from the others. For example:
- Sigma bond formed by the overlap of pure s-s, s-p, p-p (o) hybrid orbitals of two atoms along their internuclear axis. A pi bond is created by the sidewise, lateral, or parallel overlapping of two atoms’ pure “p” orbitals.
- Sigma bonds are infamous for existing independently and permit free rotation like an alkane. However, Pi-bonds must always exist alongside a sigma bond, and the rotation is regulated like alkyne or alkene.
- Sigma bonds are first made when atoms interact. Pi bonds between two atoms are usually made after sigma bonds are formed between them.
- When two given atoms are bonding, only one sigma bond is formed. But two pi bonds can easily exist between two atoms.
- Sigma bonds are renowned for having cylindrical charge symmetry around the axis of their bond, but no such symmetry exists in pi bonds.
- The electrons that form the sigma bonds are distributed in space along the axis connecting the adjoined nuclei. The electrons within the pi bonds are distributed above and below the axis but not along with it.
- Sigma bonds are often created by the pair of s orbitals in different atoms, while pi bonds combine p and similar orbitals in different atoms.
- Sigma bonds can help determine the shapes of molecules, but Pi bonds fail to serve the same purpose.
If it is a single bond, it incorporates only one sigma bond. However, double and triple bonds comprise each sigma and pi bond. Double bonds have one each, and triple bonds have one sigma bond and two pi bonds, i.e.,
1 single bond = 1 sigma bond
1 double bond = 1 sigma bond 1 pi bond
1 triple bond = 1 sigma bond 2 pi bonds
A sigma bond and a pi bond have huge differences between them, which lie in how they are created, how many atoms they need, how they move, and how they work. Although the terms sigma and pi bonds describe specific aspects of covalent bonds and molecules, they’re not interchangeable.