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Sigma and Pi Bonds

Sigma and Pi Bonds

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When you hear the words sigma and pi bond, you might think of Greek life in college. But actually, sigma and pi bonds are types of covalent bonds. Covalent bonds happen when atoms share electrons. Sigma bonds (σ) are the first type of covalent bond, formed by overlap of atomic orbitals head-to-head. They are found in single, double, and triple bonds. Pi bonds (π) are the second and third types of covalent bonds, formed by overlap of p orbitals side-to-side. They only exist in double and triple bonds.

So, what's the difference between sigma and pi bonds? First, sigma bonds are stronger than pi bonds. Second, sigma bonds can exist independently in single bonds, while pi bonds must coexist with a sigma bond and are only found in double and triple bonds.

To understand sigma and pi bonds, you need to know a little about atomic orbitals and hybridization. Atomic orbitals are spaces where electrons are likely to be found. There are four types of atomic orbital sets: s, p, d, and f. When two molecules bond, their orbitals usually combine to form hybrid orbitals like sp, sp2, and sp3.

In summary, sigma and pi bonds are types of covalent bonds formed by different types of atomic orbital overlap. Sigma bonds are stronger and can exist independently in single bonds, while pi bonds must coexist with a sigma bond and are only found in double and triple bonds.

Differences between sigma and pi bonds

 

Formation of Sigma and Pi Bonds

Right, so now you are probably wondering what head-to-head and side-to-side overlap of atomic orbitals even means.  It has absolutely nothing to do with any actual heads but instead this difference refers to where the bonding between orbitals actually occurs. In sigma bonds, head-to-head overlap means that the two orbitals are overlapping directly between the nuclei of the atoms while side-to-side means that the two orbitals are overlapping in a parallel fashion in the space above and below the nuclei.

 

Three types of sigma bonds between s-s, s-p, and p-p atomic orbitals and a pi bond between p-p orbitals
Three types of sigma bonds between s-s, s-p, and p-p atomic orbitals and a pi bond between p-p orbitals

Strength of Sigma and Pi Bonds

Now that we know what sigma and pi bonds are, let's take a closer look at some examples in different molecules.

In a molecule of H2, there is only one sigma bond. The two atoms share one pair of electrons in a sigma bond. In a molecule of O2, there are two sigma bonds and two pi bonds. The two oxygen atoms share two pairs of electrons in two sigma bonds, and two more pairs of electrons in two pi bonds. In a molecule of ethene (C2H4), there is one sigma bond and one pi bond between the two carbon atoms, and one sigma bond between each carbon atom and its two hydrogen atoms. In a molecule of acetylene (C2H2), there is one sigma bond and two pi bonds between the two carbon atoms, and one sigma bond between each carbon atom and its one hydrogen atom. As you can see, different molecules have different types and numbers of sigma and pi bonds. Understanding these bonds can help us better understand the properties and behavior of different molecules.

Examples of Sigma and Pi Bonds

The diagram above shows that sigma bonds can occur between the overlap of two s atomic orbitals, one s orbital and one p orbital or two p orbitals. Another type of interaction that creates sigma bonding is the overlap of two hybridized atomic orbitals such as sp-sp.  Pi bonds are usually formed exclusively by the side-to-side overlap of non hybridized p orbitals.  Here is a handy table below that provides examples of each type of interaction! Type of Bond Overlapping Atomic Orbitals

Example Moleculessigmas-sH2, H-Hsigmap-pF2, F-F sigmahead on head s-pHCl, H-Clsigmasp2-sp2C=C in C2H4pi bondsside to side p-pO=O in O2Table 2.

Examples of sigma and pi bonds. Source
Examples of sigma and pi bonds. Source

We are now going to explore some examples of sigma and pi bonds in the context of multiple bonds and identify how many sigma and pi bonds exist in double and triple bonds.Sigma and Pi Bonds in Double BondsSome examples of molecules with double bonds  are listed belowO2 or O=ONO or N=OCO2 or O=C=O

Double bonds occur between two atoms that share four electrons (two electron pairs).

If you said one sigma bond and one pi bond, you are correct! A double bond is always made up of one sigma bond and one pi bond. But why is this the case?

A single bond is always a sigma bond and two sigma bonds cannot exist between the same atoms. Once a sigma bond is formed with head-to-head overlap,  the only other way for two atoms to share electrons is through the side-to-side overlap of a pi bond.

Sigma and Pi Bonds in Triple Bonds

Some examples of molecules with triple bonds are listed below

N2 or C2H2 or H -  - HCO or

Triple bonds occur between two atoms that share six electrons (three electron pairs). How many sigma and pi bonds exist in a triple bond? If you said one sigma bond and two pi bonds, you are correct again! A triple bond is always made up of one sigma bond and two pi bonds.

Counting Sigma and Pi Bonds Practice Problems

Now that we know what sigma and pi bonds are and when they appear in single, double, and triple bonds, the only thing left is to put our knowledge into action!

When a question deals with counting how many sigma and pi bonds are present in a certain molecule, it may give you a condensed version of the structural formula or a full Lewis structure. If you are only given a condensed formula, you need to make sure you can accurately draw the Lewis diagram yourself. If you need a refresher, check out Lewis Dot Diagram.

Let's do a couple of examples!

How many sigma (σ) and pi (π) bonds are found in the molecule below? The good news is that this example provides us with the complete Lewis diagram, so all we need to do is count up the number of single, double, and triple bonds. There are 11 single bonds, 1 double bond, and 0 triple bonds. Remember, every single bond is a sigma bond and every double bond consists of 1 sigma bond and 1 pi bond. So, this means that in total, there are 12 sigma bonds (11 single bonds  + 1  sigma bond from the double bond) and 1 pi bond in this molecule.

Now, we will do an example where we need to draw the Lewis diagram for the molecule ourselves. It will give you practice drawing Lewis structures and counting the bonds.

How many sigma and pi bonds are found in C2H2, ethyne? The first thing we need to do is draw our Lewis structure so that we can see all the bonds properly. The correct structure should look like the following: Now, we follow the same process and count all single, double and triple bonds in the molecule. There are 2 single bonds and 1 triple bond. So, what do you think the total number of sigma and pi bonds are? There are 3 sigma bonds (2 single bonds + 1 sigma bond from the triple bond) and 2 pi bonds (from the triple bond).

When counting sigma and pi bonds in a molecule, remember that every single bond is a sigma bond, and every double bond consists of 1 sigma bond and 1 pi bond. For triple bonds, there is 1 sigma bond and 2 pi bonds. It's important to be able to draw Lewis structures accurately to count the number of bonds in a molecule. With practice, you'll become more confident in identifying and counting sigma and pi bonds in different molecules.

Sigma and Pi Bonds

How do you count sigma and pi bonds?

To count sigma and pi bonds, draw the Lewis dot structure and count the single, double and triple bonds present. Every single bond is 1 sigma bond, every double bond has 1 sigma and 1 pi bond, and every triple bond has 1 sigma bond and 2 pi bonds. With this information, you can easily count sigma and pi bonds. 

How do you identify sigma and pi bonds? 

To identify sigma and pi bonds, look at whether it's a single, double, or triple bond. Sigma bonds are always the first bond to form so every single covalent bond is a sigma bond. Pi bonds are the second and third bonds to form so double and triple bonds have the initial sigma bond and then one and two pi bonds, respectively. 

What are sigma and pi bonds?

Sigma and pi bonds are two types of covalent bonds formed by the overlapping of atomic orbitals. Sigma bonds form by the direct head to head overlap of atomic orbitals and can occur between s-s, p-p and s-p orbitals.  Pi bonds form by the side to side overlap of p orbitals. 

What is the difference between sigma and pi bonds? 

The main differences between sigma and pi bonds has to do with their formation and strength. Sigma bonds are formed by direct head to head overlap between orbitals while pi bonds are formed by side to side overlap, usually between p orbitals. This difference in formation leads to a difference in strength.  Sigma bonds are stronger than pi bonds because the direct head to head overlap provides a bigger (and therefore stronger) overlap than the side to side overlap of pi bonds. Additionally, sigma bonds form single bonds and can exist without a pi bond present; however, a sigma bond must already be formed for a pi bond to form.

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