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Sigma and Pi Bonds

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Sigma and Pi Bonds

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.

  • 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 bondWeaker covalent bond
Can exist independently in single bonds. Also found in double and triple bondsMust coexist with a sigma bond and found only in double and triple bonds

Table 1. Differences between sigma and pi bonds, Source: Tallya Lutfak, StudySmarter 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.

Sigma and Pi Bonds Diagrams of Sigma and Pi Bonds StudySmarterThree types of sigma bonds between s-s, s-p, and p-p atomic orbitals and a pi bond between p-p orbitals. Tallya Lutfak, StudySmarter 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 BondOverlapping Atomic Orbitals Example Molecules
sigmas-sH2, H-H
sigmap-pF2, F-F
sigmahead on head s-pHCl, H-Cl
sigmasp2-sp2C=C in C2H4
pi bondsside to side p-pO=O in O2

Table 2. Examples of sigma and pi bonds. Source: Tallya Lutfak, StudySmarter 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
  • C2H2 or H - - H
  • CO 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?

Sigma and Pi Bonds Lewis structure of C3H7NO2 StudySmarterLewis 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:

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.

Frequently Asked Questions about 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. 

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. 

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. 

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.  

A pi bond is formed due to the side-to-side overlap orbitals. This means the two orbitals overlap above and below the nuclei in a parallel fashion. A pi bond is only formed. It is specifically formed between two p orbitals. 

Final Sigma and Pi Bonds Quiz

Question

True or False


A sigma bond is the first type of covalent bond between atoms and is formed by side to side overlap of p orbitals

Show answer

Answer

True

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Question

Which of the following are important differences between sigma and pi bonds (choose all that apply)? 

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Answer

Sigma bonds are weaker than Pi bonds

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Question

How many sigma and pi bonds exist in a single covalent bond? 

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Answer

1 sigma bond. Every single bond is made up of 1 sigma bond.

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Question

If there are 2 pi bonds and 1 sigma in a bond, what type of bond is it? 

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Answer

Single Bond

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Question

How many sigma and pi bonds are found in O2?

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Answer

There is a double bond in O so there is 1 sigma bond and 1 pi bond.

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Question

How can sigma bonds form?

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Answer

side to side overlap between p orbitals 

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Question

Count the number of sigma and pi bonds in the following molecule:

 

O = C = O

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Answer

There are 2 sigma bonds and 2 pi bonds

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Question

How many pi bonds are there in a double bond? 

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Answer

0

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Question

How many sigma and pi bonds are there in PCl5?



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Answer

0 sigma bonds and 5 pi bonds

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Question

How many sigma bonds are there in a triple bond?

Show answer

Answer

1 sigma bond

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Question

How many sigma and pi bonds are in a double bond? 

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Answer

1 sigma bond and 1 pi bond

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Question

How do pi bonds form? 

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Answer

Side to side overlap of p orbitals 

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Question

Sigma and pi bonds are what types of bond? 

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Answer

Covalent

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Question

How many pi bonds are there in water molecules? 

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Answer

0. There are only single bonds between the hydrogen and oxygen atoms so there are no pi bonds. 

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