Lewis Dot Structures: Sigma & Pi Bonds: Videos & Practice Problems

In chemistry, a covalent bond involves the sharing of two valence electrons between atoms, forming a strong connection known as a sigma bond (σ-bond). Sigma bonds are the primary bonds that directly link atoms in a molecule. Pi bonds (π-bonds), on the other hand, are weaker and serve to reinforce the sigma bond, much like a phone cover protects the phone. While a single bond consists of one sigma bond and no pi bonds, a double bond includes one sigma and one pi bond, and a triple bond has one sigma bond with two pi bonds. As the number of pi bonds increases, the overall bond strength increases, but the bond length decreases. This means that a triple bond is stronger and shorter than a double bond, which in turn is stronger and shorter than a single bond. Understanding the relationship between bond types, strength, and length is crucial for grasping molecular structure and stability.

Lewis Dot Structures: Sigma & Pi Bonds

A single covalent bond is formed by the sharing of two valence electrons between two elements, with each element contributing one electron. This results in a strong connection between the atoms. In the context of covalent bonding, it is essential to understand the distinction between sigma (σ) and pi (π) bonds. A sigma bond is the strongest type of covalent bond, directly linking the two atoms, while a pi bond is weaker and serves to insulate and protect the sigma bond.

As the number of pi bonds increases between two elements, the overall bond strength increases, but the bond length decreases. For instance, in a single bond, there is one sigma bond and no pi bonds. In a double bond, there remains one sigma bond, but now there is one pi bond, which adds a layer of protection to the sigma bond. This can be likened to a cell phone: the sigma bond is the solid phone itself, while the pi bond is akin to the protective cover around it.

When examining triple bonds, the structure remains consistent with one sigma bond, but there are now two pi bonds. This configuration provides even greater protection for the sigma bond, resulting in a stronger overall bond. The relationship between bond strength and length is crucial: single bonds are the longest, double bonds are shorter and stronger due to the additional pi bond, and triple bonds are the shortest and strongest, with the sigma bond being well-insulated by the two pi bonds.

In summary, as the number of pi bonds increases, the bond strength enhances while the bond length diminishes, illustrating the trade-off between these two characteristics in covalent bonding.

Lewis Dot Structures: Sigma & Pi Bonds Example 1

In the study of chemical bonds, understanding the relationship between bond length and bond strength is crucial. Bond strength refers to the energy required to break a bond between two atoms, while bond length is the distance between the nuclei of those atoms. It is essential to recognize that there is an inverse relationship between these two properties: as bond strength increases, bond length decreases. This means that stronger bonds are typically shorter, as the atoms are held together more tightly.

For example, in a series of compounds involving bonds between the same two atoms, if one bond is stronger than another, it will be shorter in length. This relationship is consistent across various types of bonds, including single, double, and triple bonds. A single bond is generally longer and weaker compared to a double bond, which is shorter and stronger. Therefore, when evaluating statements about bond length and bond strength, it is important to remember that they are inversely proportional. The correct interpretation is that greater bond strength corresponds to shorter bond length, reinforcing the idea that these two properties are fundamentally linked in a predictable manner.

How many pi bonds does the following molecule contain?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (D)

How many sigma bonds does the following molecule contain?

• A. 12
• B. 8
• C. 9
• D. 10

Answer: (C)

Which has greater bond strength between the carbon–carbon bond.
C₂Cl₂  vs.  C₂Cl₆

• A. C₂Cl₂
• B. C₂Cl₆

Answer: (A)

Draw the total number of sigma and pi bonds of the sulfur trioxide molecule, SO₃.

• A. 4 sigma, 0 pi
• B. 3 sigma, 1 pi
• C. 2 sigma, 2 pi
• D. 1 sigma, 3 pi

Answer: (B)

Here’s what students ask on this topic:

What is a sigma bond?

A sigma bond (σ bond) is the strongest type of covalent chemical bond where two atomic orbitals directly overlap between the nuclei of two atoms. It is formed by the head-on or axial overlapping of atomic orbitals, which allows for the maximum overlap and therefore a strong bond. In a sigma bond, the electron density is concentrated along the axis connecting the two bonding nuclei.

Sigma bonds can be formed from the overlap of s orbitals with other s orbitals, s orbitals with p orbitals, or p orbitals with p orbitals, as long as the orbitals are oriented to overlap end-to-end. Each single bond in a molecule is a sigma bond, and it allows for free rotation about the bond axis because the electron density is cylindrically symmetrical around the bond axis. In contrast, pi bonds (Π bonds), which can accompany sigma bonds in double and triple bonds, do not allow for this kind of rotation due to their different orbital overlap.

What does a sigma bond look like?

A sigma bond (σ bond) is the strongest type of covalent chemical bond and is formed by the direct overlap of atomic orbitals from two different atoms. To visualize a sigma bond, imagine two spherical s orbitals or two dumbbell-shaped p orbitals from two separate atoms coming together so that their electron clouds overlap directly between the nuclei of the atoms. This head-on overlap allows for a strong bond because the electron density is concentrated along the axis connecting the two nuclei, which maximizes the attraction between the positively charged nuclei and the negatively charged electrons.

In diagrams, sigma bonds are often represented as a single line connecting two atoms, indicating a single pair of shared electrons. For example, in a molecule of hydrogen (H₂), the two hydrogen atoms are connected by a single line, representing a sigma bond formed by the overlap of their 1s orbitals.

What is a pi bond?

A pi bond (Π bond) is a type of covalent bond that forms when two p orbitals overlap in a side-by-side fashion. Unlike sigma bonds (σ bonds), which occur when orbitals overlap end-to-end and allow for free rotation of the bonded atoms, pi bonds restrict rotation because of the parallel overlap. Pi bonds are found in double and triple bonds alongside sigma bonds. In a double bond, there is one sigma bond and one pi bond, while in a triple bond, there is one sigma bond and two pi bonds.

The electron density in a pi bond is located above and below the plane of the nuclei of the bonding atoms, which gives these bonds their characteristic properties, such as the inability to rotate and the increased reactivity compared to sigma bonds. Pi bonds are less strong than sigma bonds because the overlap is not as effective, making them more susceptible to reactions. Understanding pi bonds is crucial in organic chemistry, as they play a key role in the structure and reactivity of molecules.

What does a pi bond look like?

A pi bond (Π bond) is a type of covalent bond that forms when parallel orbitals overlap and electrons are shared between atoms. Unlike sigma bonds, which are formed by end-to-end overlapping and are found in all single bonds, pi bonds are formed from the side-to-side overlap of two p orbitals.

To visualize a pi bond, imagine two adjacent atoms, each with an electron cloud shaped like a dumbbell, which represents a p orbital. A pi bond occurs when these dumbbell-shaped clouds overlap on their sides. This overlap happens above and below the plane of the atoms involved in the bond.

In diagrams, pi bonds are often represented by a line or dash parallel to the sigma bond, indicating the side-by-side connection. In a double bond, for example, you have one sigma bond and one pi bond. The sigma bond is the first bond formed and is stronger and more stable, while the pi bond is the second bond, which is weaker and gives the double bond its additional reactivity compared to a single bond.

What is the difference between a sigma bond and a pi bond?

Sigma (σ) bonds and pi (Π) bonds are types of covalent bonds that differ in the way the atomic orbitals overlap. A sigma bond is the strongest type of covalent bond and is formed by the end-to-end (head-on) overlap of atomic orbitals. Typically, this involves the s orbitals, but p orbitals and hybrid orbitals can also form sigma bonds. A single bond between two atoms consists of one sigma bond.

On the other hand, a pi bond is formed by the side-to-side overlap of two p orbitals, with their electron density located above and below the plane of the nuclei of the bonding atoms. Pi bonds are generally weaker than sigma bonds. In a double bond, there is one sigma bond and one pi bond, while in a triple bond, there is one sigma bond and two pi bonds. The presence of pi bonds causes rigidity in the molecule's structure, restricting rotation around the bond axis, which is not the case with sigma bonds.