When you hear the words sigma and pi bond, eager dreams of joining Greek life and being bonded with your Greek brothers or sisters in college may come to mind. But did you know that sigma and pi bonds are actually types of covalent bonds?
Sigma bonds (σ) are the first type of covalent bond found between two atoms formed by head-to-head overlap. They exclusively make up single bonds and are also found in double and triple bonds.
Pi bonds (π) are the second and third types of covalent bonds found between two atoms formed by side to side overlap of p orbitals. They are only found in double and triple bonds.
- This article is about the sigma and pi bonds.
- Together, we will go deeper into what sigma and pi bonds are and have a look at their differences.
- Then, we will briefly cover some examples of sigma and pi bonds.
- Afterwards, we will look at the breakdown of sigma and pi bonds in double and triple bonds.
- Finally, to apply what we have learned, we will do some practice problems in counting sigma and pi bonds.
Remember that covalent bonds form from the overlap of atomic orbitals which are just the space where electrons are likely to be found. There are several types of atomic orbital sets: s, p, d, and f. Each of these sets can hold a different amount of orbitals, exist at different energy levels, and have different shapes. When two molecules bond, the orbitals usually combine to form hybrid orbitals such as sp, sp2, and sp3. To understand Sigma and Pi bonds, you must have a basic understanding of atomic orbitals, hybridization, and hybrid orbitals. Check out the explanations for these terms if you need to review them!
Differences Between Sigma and Pi Bonds
Below is a table highlighting the most important differences you need to know between sigma and pi bonds. We will go into more detail on each one.
| Sigma Bonds (σ) | Pi Bonds (π) |
| Formed by head-to-head overlap between atomic orbitals (both hybridized and unhybridized) | Formed by side-to-side overlap between p orbitals |
| Strongest covalent bond | Weaker covalent bond |
| Can exist independently in single bonds. Also found in double and triple bonds | Must coexist with a sigma bond and found only in double and triple bonds |
Table 1. Differences between sigma and pi bonds, Source: Tallya Lutfak, Vaia originals
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. Tallya Lutfak, Vaia Original.
Strength of Sigma and Pi Bonds
As seen above, sigma bonds have a larger area of bonding overlap. Because of the difference in overlap, sigma and pi bonds differ in bonding strength. This larger area of overlap corresponds to a higher chance of finding valence electrons between the nuclei of the atoms. Additionally, the electrons are closer to the nuclei, so the sigma bond is stronger.
While a single sigma bond is stronger than a pi bond, when they are both present (such as in double and triple bonds) its combined strength is greater than that of a single bond.
Next, we shall look at some examples of sigma and pi bonds in different molecules so that you are more familiar with the orbital interactions associated with each bond.
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 Molecules |
| sigma | s-s | H2, H-H |
| sigma | p-p | F2, F-F |
| sigma | head on head s-p | HCl, H-Cl |
| sigma | sp2-sp2 | C=C in C2H4 |
| pi bonds | side to side p-p | O=O in O2 |
Table 2. Examples of sigma and pi bonds. Source: Tallya Lutfak, Vaia Original
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 Bonds
Some examples of molecules with double bonds are listed below
- O2 or O=O
- NO or N=O
- CO2 or O=C=O
Double bonds occur between two atoms that share four electrons (two electron pairs).
Remember that the first covalent bond to form between two atoms is always a sigma bond and the second and third bonds are pi bonds. So with this information, how many sigma and pi bonds do you think are found in a double bond?
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
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- C2H2 or H -
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- H - CO or
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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?
Fig. 2: Lewis Structure of C3H7NO2.
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:
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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).
Sigma and Pi Bonds - Key takeaways
- Sigma Bonds form by the head-to-head overlap of atomic orbitals and are the first covalent bonds formed between atoms.
- Pi Bonds form by the side-to-side overlap of p orbitals and are the second and third bonds formed between atoms.
- The main differences are that sigma bonds can form between hybridized orbitals and are stronger than pi bonds.
- A single bond consists of 1 sigma bond, a double bond consists of 1 sigma bond and 1 pi bond and a triple bond is 1 sigma bond and 2 pi bonds.
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