The study of bonds is an important aspect in the field of chemistry. A chemical bond is a long-term attraction between ions, molecules, or atoms that help form chemical compounds. Atoms usually have less than eight electrons in their valence shell which makes them unstable. Atoms become stable when they react with each other and have more than 8 electrons in them. These are achieved by metallic bonds, ionic bonds or covalent bonds.
Let’s conduct a Sigma vs Pi Bond analysis for better understanding.
Sigma and Pi Bonds in Covalent Bonds
Covalent bonds can make multiple bonds between two atoms. Atoms that are similar or have significantly low electronegativity differences can react by sharing electrons to form Covalent Bond. Multiple bonds can be formed in two ways – Sigma Bond and Pi bond.
Sigma bond and PI Bond are two types of covalent bonds in a molecule. For example, consider the C2H4 molecule. It has 5 sigma bonds and 1 Pi bond. Both of them are formed as a consequence of overlapping orbitals of different atoms.
The overlapping happens due to incomplete orbitals of the atoms. The overlapping can either be head-on or may happen side-to-side. This results in the formation of Sigma bond and Pi Bond.
Sigma Bond
Sigma bond is represented by the Greek symbol (σ). It is the strongest bond where atoms are formed by the overlap s or p orbitals in an end-to-end manner, and the electron density gets concentrated between the nuclei of the bonding atoms.
Sigma bond is symmetrical with regards to rotation around the bond axis. s+s, pz+pz, s+pz and dz2+dz2 are common forms of sigma bond (z denotes the axis of the bond). These bonds do not show hyper-conjugation or any bond resonance.
The below picture shows Sigma bonds and depicts different types of overlap formed by head-on overlapping of atomic orbitals.
Pi Bond
Pi Bond is represented by the Greek symbol (π), which refers to P orbitals. The overlap of p orbitals forms this bond in a side-by-side manner, and the electron density gets concentrated above and below the nuclei of the bonding atoms.
Two lobes of an orbital overlap in Pi Bond, with two lobes of another orbital of another atom. This helps in lateral overlap. Pi bonds can form double or triple bonds but never a single bond. Pi Bonds tend to demonstrate properties of the conjugated structure.
In the below picture, an illustration of how two p-orbitals are forming a Pi bond with a side-by-side overlap has been shown.
Differences between Sigma Bond & PI Bond:
It is important to distinguish Sigma vs Pi bond and know the comparative properties of both of these bonds. While the variances between the sigma bond and Pi bond are complimentary, the concrete differences have mentioned in the table below for a better understanding.
| Sl No. | Sigma (σ) Bond | Pi ((π)) Bond |
| 1 | Sigma bonds are the strongest covalent bonds due to a huge extent of overlapping. | They are weaker than Sigma bonds due to the lesser extent of overlapping. |
| 2 | Sigma bonds are highly reactive | Pi bonds are less reactive than the atoms that have sigma bonds |
| 3 | It is formed by end-to-end, meaning head-on collision of the atomic orbitals | The side-by-side overlap of atomic orbitals forms it |
| 4 | When atoms interact, Sigma Bonds are formed | Pi bonds are formed after sigma bonds are formed |
| 5 | These are formed between any two orbitals, for example, s-s, s-p or p-p | These can be formed only between ‘p’ orbitals |
| 6 | The shape of a molecule is determined by the Sigma bond. It results in higher electron density between two nuclei on the internuclear axis | It does not have any effect on the shape of the molecule, direction of the bond or the internuclear distance |
| 7 | They exist independently | They co-exist along with Sigma bond |
| 8 | There is free rotation of atoms around the sigma bond | Free rotations of atoms are not possible around the Pi bond as they involve in the breaking of Pi bonds |
| 9 | They consist of a single charge cloud as the molecular orbital is symmetrical around the internuclear axis. | They consist of two charged clouds placed above and below the plane of the atom, and their molecular orbital is non-symmetrical and discontinuous |
| 10 | The lobe that participates in the overlapping gets stretched while the other lobe reduces. | Both of the participating lobe’s bond and continue to remain of the same size |
| 11 | The stability of the compound is directly proportional to the number of sigma bonds present | The stability of the compound is inversely proportional to the number of Pi bonds present |
| 12 | The relativity of the compound is inversely proportional to the Sigma bonds | The relativity of the compound is directly proportional to the Pi bonds |
| 13 | Doubled and triple bonds have only one sigma bond | A double bond had one Pi bond, while a triple bond had two Pi bonds |
| 14 | Sigma bonds are involved in controlling the geometrical polyatomic molecules | Pi bonds are not involved in controlling the geometrical polyatomic molecules |
| 15 | Orbitals that overlap in a sigma bond are either be one hybrid or one pure orbit or between both hybrid and both pure orbitals | Orbitals that overlap in a Pi bond takes place between two non-hybrid orbitals |
| 16 | Example: Methane has the chemical formula CH₄, here C-H bonds in methane are Sigma Bonds | Example: Ethene has the chemical formula C2H4, here C-C double covalent bonds in Ethene has one Sigma Bond and other Pi Bonds |
Conclusion
To summarize, Sigma Bonds and Pi Bonds occur from the same origin, and both are covalent chemical bonds. Sigma bond is between atoms inside a molecule formed by S orbitals that overlap on the axis joining the nuclei. On the other hand, Pi Bonds are molecular bonds formed by overlapping P orbitals from diverse atoms.
The massive differences between them, their formation requirement, presence of atoms, their movement and functionality make them distinct from each other and cannot be ignored. It is important to understand and know the differences between the features and characteristics of Sigma Bond and Pi Bond.
