Difference Between Sigma and PI Bonds – Characteristics, Types and Strength of Sigma and PI Bonds, and Their Differences

Bonding always creates a connection between two entities. Be it among humans or atoms. A connective link is what establishes a secure relationship! For example, a warm handshake between two people meeting for the first time initiates a beautiful start. Or two hands joined to convey a ‘Namaste’ is a similar way of traditional greeting. These two methods of greeting can be easily thought of as analogous to the two specific types of covalent bonds – Sigma and pi bonds.

A handshake is a widely practised, quick welcome or farewell custom that involves two individuals grabbing each other's similar hands and, in most situations, briefly moving them up and down. It signifies a head-on orbital overlap just as is the case in sigma () bonds. Whereas the act of saying a ‘Namaste’ can be thought of as the sidewise overlapping of orbitals that generates a pi () bond.

The strength of these two patterns of bonds (-bond and -bond) can also be correlated with these two customs of greeting. One has a higher degree of close contact and overlap and so has a greater strength (handshake for -bond!), while the other signifies a lateral overlap of two hands touching each other laterally signifying a lesser strength (Namaste for -bond).

Let’s understand both types in a bit more detailed manner and find out the major differences between the two.

TABLE OF CONTENTS

• Chemical Bonds – Introduction
• Sigma Bond – Definition
• Sigma Bond – Characteristics
• Sigma Bond – Types
• Pi Bond – Definition
• Pi Bond – Characteristics
• Pi Bond – Types
• Strength of Sigma and Pi Bonds
• Differences between Sigma and Pi Bonds
• Practice Problems
• Frequently Asked Questions – FAQ

Chemical Bonds – Introduction

A force that holds atoms together to create molecules or compounds is known as a chemical bond. There are many different kinds of chemical bonds, including ionic, covalent, hydrogen, and coordinate bonds.

• Ionic bonds are created when the valence electrons are transferred between two charged ions. The electrostatic interactions between the charged particles are primarily what creates an ionic bond. It is also known as an electrovalent bond as a result.

• Covalent bonds are those created when two atoms share an identical pair of electrons. Covalent bonds entail the sharing of electron pairs between atoms.

• A hydrogen bond is the name given to the frail link created between an atom's proton and another electronegative atom.

• A coordinate bond is a link created between two atoms when one atom provides a pair of electrons and the other accepts the same pair. One atom serves as the donor and the other as the acceptor in a coordinate bond.

Sigma bonds and pi bonds are two different forms of covalent chemical bonding. Covalent bonds include the sigma and pi bonds. Sigma and pi bonds are fundamental to many other chemical topics, therefore understanding them is necessary. Let's examine how these bonds are created and what the main distinctions are between the two.

Sigma Bond – Definition

Sigma bonds are generated formed when hybrid orbitals overlap with each other along the bonding axis. This is basically a head-on overlap of orbitals that generate sigma bonds. The shared electron density of the two atoms in a sigma bond is located precisely along the bonding axis.

The resulting relationship is significantly stronger and more stable. The sigma bond is the first covalent bond that forms when two distinct atoms interact. Sigma bonds are followed by the development of pi bonds.

• Sigma bonds, often known as - bonds, are the strongest kind of covalent chemical bonding in chemistry.

• They are formed by the head-on overlap of one atomic orbital with another.

• A - bond is symmetrical with regard to rotation about the bond axis.

• s+s, pz+pz , s+pz, and dz2+dz2 are typical types of sigma bonds (where z is defined as the axis of the bond or the internuclear axis, s, p and d are the orbitals).

• A single bond is often a sigma () bond, and a multiple bond is made up of a sigma bond together with pi () or other bonds.

• One sigma () plus one pi () bond makes up a double bond, while one sigma () plus two pi () bonds make up a triple bond.

Sigma Bond – Characteristics

• The sigma bond is created by the hybrid orbitals overlapping directly over one another (head-on overlap).

• Sigma bonds are stable and strong.

• When two atoms or molecules interact with one another, the first thing that happens is the development of a sigma bond.

• Typically, the symbol ‘’ is used to represent the sigma bond.

• Sigma bonds form in all alkanes, alkenes, and alkynes.

Sigma Bond – Types

s-s Overlapping: One 's' orbital from each participating atom experiences head-on overlapping along the internuclear axis in this type of overlapping. Before one s-orbital can overlap with another, it must be half-filled. Each hydrogen atom in a H2 molecule has a half-full ‘s’ orbital, which leads to this kind of overlap.

s-p Overlapping: Here, along the internuclear axis, one half-filled s orbital and one half-filled p orbital overlap and create a covalent bond.

In ammonia, this kind of overlap can be seen. The 2px, 2py, and 2pz orbitals of the nitrogen atom overlap with the 1s orbitals of the three hydrogen atoms to create the three sigma bonds that make up an NH3 molecule.

p-p Overlapping: One half-filled p orbital from each involved atom experiences a head-on overlapping along the internuclear axis in this situation.

The 3pz orbitals of two chlorine atoms overlap in a p-p head-on overlap to form a Cl2 molecule. The development of a sigma bond results from the head-to-head overlap of two p orbitals, whereas the formation of pi bonds results from the lateral overlap of these orbitals.

Pi Bond – Definition

Pi bonds are covalent chemical bonds in which two lobes of one atomic orbital laterally overlap with two lobes of an atomic orbital that belongs to a different atom. Pi bonds are frequently expressed as "-bonds" where the Greek character "" corresponds to the similar kind of symmetry of the p-orbitals and the -bond formed.

-bonds are covalent bonds which are present when two atoms undergo double or triple bonding. Although d orbitals also participate in - bonding, p orbitals are frequently involved in this kind of bonding.

• Due to the p-orbitals' parallel orientation, there is substantially less overlap between them, which explains why this connection is weak from a quantum mechanical standpoint.

• Sigma bonds, in contrast, generate bonding orbitals right between the connecting atoms' nuclei, resulting in more overlap and a strong sigma bond.

• Atomic orbitals that are in touch through two regions of overlap cause -bonds. Compared to sigma bonds, -bonds are more diffused.

• -electrons are occasionally used to refer to electrons in pi () bonds. Because rotation implies removing the parallel alignment of the constituent p orbitals, molecular fragments linked by a pi () bond cannot rotate about that connection without rupturing the pi() bond.

Pi Bond – Characteristics

• -bonds are created by the sideways overlapping of two p-orbitals from nearby atoms that are parallel in orientation.

• The extent of overlapping in -bonds is less than that in sigma bonds because it occurs at the side of the two lobes of p-orbitals. -bonds are hence weaker than -bonds.

• In -bonds, the area perpendicular to the bond axis has a high concentration of electrons.

• The -bond's molecular orbital is orientated above and below the plane that the nuclear axis occupies.

• If the -bond forms in the molecule, all atoms connected to the atom forming the pi bond would be on the same plane.

• Between two atoms, only one -bond can be formed, the rest are π-bonds.

• One π-bond and one -bond make up double covalent bonds, whereas one -bond and two π-bonds make up triple covalent bonds. So, it must be noted that if just one covalent link exists between the atoms, it will always be a sigma covalent bond.

Pi Bond – Types

-bonds between two atoms are formed in addition to a sigma bond. It is always present in the molecules containing multiple bonds (double or triple bonds).

There are three types of orbital overlaps which can generate -bonds.

• p-p bond forms between the p-orbital of one atom and the p-orbital of the other atom. Example: C2H4, C2H2

• p-d overlap involves the p-orbital of one atom with the d-orbital of the other atom.

• When there is a bond between two atoms with one atom having one vacant orbital and the other has one lone pair of electrons, and if this electron pair is donated to that corresponding vacant orbital then the bonding is called p-p or p-d depending on the orbital to which the electron pair is being donated to and from which orbital the electron pair is coming.

• For example: Nitrogen (N2) is unable to form p-d bond because of the absence of vacant d-orbitals in its outermost shell. But elements which show extended covalency are mainly due to the presence of vacant d-orbitals. For example, Phosphorus (P4), can form p-d bonds.

• Another case is of SO2 molecule, in which S atom undergoes sp2 hybridisation. The multiple bonding in this molecule is due to p-d bonding. The vacant 3d-orbitals of S overlap with the filled 2p- orbitals of O.

• d-d is a form of pi-bonding in molecular species where multiple bonds are seen. In this a central atom with a p or d-valence shell participates in bonding with species having empty, partially or completely filled d-orbitals. This results in the formation of p-p or p-d bonding.

• p-p or p-d overlap is formed by sidewise or lateral overlap, in addition to the -overlap formed by direct head-on overlap.

• In case of d-d overlap, filled d-orbitals of the atom donate electrons and laterally overlaps with the vacant d-orbital of the surrounding atoms. Example: R3P, R3As, R2S, etc,.

Strength of Sigma and Pi Bonds

The degree of overlap affects how strong a bond is. The bond is stronger when there is greater overlap.

Because of the spherical charge distribution in the s-orbital, it is non-directional. The charge distribution in the p-orbital is directed. As a result, the sigma bond created by the overlap of two p orbitals will have a greater degree of overlap than the bonds created by the overlap of s-p orbitals and s-s orbitals.

-bonds (sigma bonds) are generally stronger than -bonds. For instance, a C-C single bond has an energy that is two times larger than a C=C double bond with one - and one -bond. The quantum mechanical viewpoint may be used to explain why these bonds are less strong than sigma bonds.

This viewpoint claims that the degree of p-orbital overlap in -bonds are noticeably smaller, due to their parallel alignment. In contrast, sigma bonds have a significantly greater level of overlapping due to head-on overlap leading to a greater extent of effective overlap. As a result, they are often stronger than comparable bonds.

The order of strength of -bonding for same shell orbitals (considering the same value of n for all) follows the below order: d-d>p-d>p-p

The strongest bonding is found in d-d followed by p-d bond which is stronger than the p-p bond.

The strength of the bond is inversely proportional to the orbital size and directly proportional to extent of overlap. The bent shape of two adjacent d-orbitals of the same shell creates stronger attraction between nucleus and electrons and creates a strong overlap.

In p-p bonding, the overlapping lobes of p-orbitals are parallel to each other, which results in a pure sideways (close to 180overlap). Whereas in p-d the overlapping lobes of d-orbitals are rather closer (<180) to the lobes of p-orbitals. Due to this reason, the overlap is stronger in the case of p-d if compared with p-p.

The relative order of strength of orbital overlap is given by the following order:

2pπ-2pπ>2pπ-3dπ>2pπ-3pπ>3pπ-3pπ

The strength of π-bond is directly proportional to the effective overlap of orbitals. As greater overlapping occurs, bond strength increases.

Differences between Sigma and Pi Bonds

Sigma () Bond: In this type of covalent bonding, the atomic orbitals that are half-filled overlap head-on along the internuclear axis to create the sigma () bond. The overlap may also be referred to as axial overlap or head-on overlap. Sigma electrons are the electrons that make up a sigma bond. Two hybrid orbitals, one hybrid and one pure orbital, or two pure orbitals may overlap in sigma bonds.

Pi () Bond: This particular kind of covalent bond is created by the side-by-side or lateral overlap of partially filled pure atomic orbitals. The atomic orbitals overlap so that their axes continue to be perpendicular to the internuclear axis and parallel to one another.

The following are a few significant differences between sigma and pi bonds.

Below is an example of two molecules, O2 and N2, where both these two types of bonds are exhibited.

Practice Problems

1. The number of sigma and pi bonds in the enolic form of ethyl acetoacetate are:

1. 18 sigma bonds and 2 pi bonds
2. 9 sigma bonds and 1 pi bond
3. 9 sigma bonds 2 pi bonds
4. 16 sigma bonds and 1 pi bond

Answer: A

Solution: Let us first take a look at the enolic form of ethyl acetoacetate:

As we can see there are two double bonds. Each double bond denotes 1 -bond and 1 - bond. This implies that there are 2 pi () bonds. Rest are all single bonds. So, there are 18 -bonds and 2 -bonds.

So, option A is the correct answer.

2. Why is the overlapping of 2pπ-2pπ>2pπ-3dπ ?

Solution: The overlapping of 2pπ-2pπ orbitals is stronger than 2pπ-3dπ, as the energy gap is lower between them (as principle quantum numbers for both are the same, n=2).

The important comparison is between 2pπ-3dπ and 2pπ-3pπ. The strength of overlap between 2pπ-3dπ is more than 2pπ-3pπ because the 3p-orbitals are less bent than two lobes of 3d-orbitals.

3. Sigma and pi bonds are two types of:

1. Covalent Bonds
2. Coordinate Bonds
3. Hydrogen Bonds
4. Ionic Bonds

Answer: A

Solution: Sigma and pi bonds are basically covalent bonds formed by the overlap of orbitals. Atomic orbitals overlap to form covalent bonds. While pi bonds are created by the lateral overlapping of two atomic orbitals, sigma bonds are created by the head-to-head overlapping of atomic orbitals.

So, option A is the correct answer.

4. A covalent bond is formed by the overlap of:

1. Valence Shells
2. Nucleus
3. Electrons
4. Atomic Orbitals

Answer: D

Solution: In accordance with the valence bond theory, a covalent bond is formed by the overlapping of atomic orbitals, each containing a single electron of opposite spin.

So, option D is the correct answer.

Frequently Asked Questions – FAQ

1. Are sigma and pi bonds similar?
Answer: Sigma bonds and pi bonds are not similar. A pi bond is created by the parallel or lateral overlapping of the atomic orbitals, whereas a sigma bond is formed by the linear or co-axial overlap of the atomic orbitals of two atoms.

2. Why do orbitals overlap?
Answer: An orbital is the region surrounding the nucleus where the chances of finding electrons are maximum. The orbitals overlap to form bonds and thereby making the bonding atoms stable.

3. Which is more strong as a bond - sigma or pi bond?
Answer: In pi bonds, there is less overlap than in sigma bonds due to lateral overlap. Sigma bonds are therefore more powerful than pi bonds due to more close contact and greater extent of overlapping. Note that bond length affects bond strength. The head-to-head overlap creates the sigma connection.

4. Is -bond equivalent to a double bond?
Answer: A double bond consists of a sigma and a pi bond. Therefore, a pi bond is not equivalent to a double bond. The presence of a double bond denotes that it definitely has a pi bond. Hence, -bond is a major identity of a double bond. A double bond has to have a -bond.

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