When you hear the words “sigma bind” and “pi bind,” you might think of being in college and getting close to your Greek brothers and sisters. But did you know that sigma bonds and pi bonds are also covalent bonds?
What is a Sigma Bond?
When two atoms overlap head-to-head, they form sigma bonds (σ), which are the first type of covalent bond. They always have a single bond, but they can also have double or triple bonds.
What is a Pi Bond?
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.
Key Differences Between Pi and Sigma Bond
|Components||Pi Bond||Sigma Bond|
|Formation||This bond is formed by the two Sigma Bonds||Sigma Bond formation takes place by overlapping atoms|
|Reaction||A Pi Bond is not as much reactive as a Sigma Bond||Sigma Bond is highly reactive|
|Overlapping||Pi Bond has side by side overlapping of atomic orbitals||Sigma Bond has a face to face overlapping of atomic orbitals|
|Independence||A Pi Bond is not free and independent. It always wanders around the Sigma Bond||Sigma Bond is a free and independent bond|
|Symmetry||Pi Bond does not have any symmetry||It has a cylindrical symmetry around the axis of the bond|
|Molecule Shape||This bond does not determine the shape of the molecule||The shape of the molecule is determined by this bond|
Pi vs. Sigma Bonds Similarities
Examples of Sigma and Pi Bonds
As shown in the picture above, sigma bonds can form when two s orbitals, one s orbital and one p orbital, or two p orbitals overlap. In sigma bonding, two hybridized atomic orbitals, such as sp-sp, can touch each other.
This is another type of contact. Pi bonds are often only made when two non-hybridized p orbitals touch from side to side. Below is a helpful table with examples of each type of interaction.
|Type of Bond||Overlapping Atomic Orbitals||Example Molecules|
|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|
Now, we’ll look at some examples of sigma and pi bonds in multiple bonds and figure out how many sigma and pi bonds there are in double and triple bonds.
Sigma and Pi Bonds in Double Bonds
Here are a few examples of molecules that have double bonds.
NO, N=O CO2, O=C=O O2, or O=O
When two atoms share the same four electrons, they form a double bond (two electron pairs).
From this information, how many sigma and pi bonds do you think a double bond has?
You are right if you said one sigma bond and one pi bond. A double bond is made up of one sigma bond and one pi bond. But how can that be?
One sigma bond is always possible, but two between the same atoms are never possible. The only way for two atoms to share electrons after making a sigma bond with head-to-head overlap is to make a pi bond with side-to-side overlap.
Sigma and Pi Bonds in Triple Bonds
Here are some examples of molecules that have three bonds between them.
- N2 or
- C2H2 or H – – H
- CO or
Triple bonds are made when two atoms share six electrons (three electron pairs).
Counting Sigma and Pi Bonds Practice Problems
Now that we know what sigma and pi bonds are and when they happen in single, double, and triple bonds, the only thing left to do is put our knowledge to use!
When asked how many sigma and pi bonds are in a certain molecule, the answer could be a shortened version of the structural formula or a full Lewis structure.
If you are only given a short formula, you must be sure you can draw the Lewis diagram correctly. The Lewis Dot Diagram can help you remember what you already know.
Pi vs. Sigma Bond Pros and Cons
Pros and Cons of the Pi Bond
Pros of the Pi Bond
- In MO theory, a Pi bond is used to figure out how molecules will act.
- It is a necessary covalent bond that holds the atoms together and helps the molecule form.
Cons of the Pi Bond
- A Pi bond is a weak bond that is not independent because it always circles the Sigma bond.
- Since a Pi bond is not a stable bond, it is hard to predict bond energy and other things about it.
Pros and Cons of a Sigma Bond
Pros of a Sigma Bond
- Compared to the Pi bond, the Sigma bond is the strongest and most reactive.
- Sigma Bond is a bond that is stable enough to handle its own instability.
Cons of a Sigma Bond
- When you compare how reactive Sigma Bond and Pi Bond are, Sigma Bond is less reactive.
- Sigma bonds can only be made when the orbitals of two atoms touch each other.
Frequently Asked Questions about Sigma and Pi Bonds
How do you count sigma and pi bonds?
To count the number of sigma and pi bonds, make a Lewis dot structure and count the number of single, double, and triple bonds.
How do you identify sigma and pi bonds?
Look for single, double, or triple bonds to find sigma and pi bonds. Sigma bonds are always the first to form, so they make up all covalent bonds.
Because p-bonds come after sigma bonds, double and triple bonds start with a sigma bond and then one or two p-bonds.
What are sigma and pi bonds?
They are made when two atomic orbitals overlap directly head-to-head. When p orbitals overlap side by side, pi bonds develop.
What is the difference between sigma and pi bonds?
Sigma and pi bonds are very different in how they form and how strong they are.
Sigma bonds are made when two orbitals overlap head-to-head, while pi bonds are made when two orbitals overlap from the side. The strength changes because of this change in shape.
Sigma bonds are stronger than pi bonds because the overlap between the two ends of a sigma bond is larger (and therefore stronger) than the overlap between the two ends of a pi bond.
How a pi bond is formed?
When two orbitals overlap from side to side, a pi bond is made. This means that the two orbitals are in the same place above and below the nucleus. It only makes a Pi connection. It is made right in the middle of two p-orbitals.
When atomic orbitals overlap, they form sigma bonds, which are the first covalent connections between atoms.
When p orbitals touch each other from side to side, they make pi bonds. These are the second and third bonds that form between atoms.
Instead of pi bonds, which can’t form between hybridized orbitals, sigma bonds can and are stronger when they do.
One sigma bond, one pi bond, and one double bond make up a single bond. One sigma bond and two pi bonds make up a triple bond.