A pi bond is a chemical bond, while pi electrons are the electrons within that pi bond. Think of it like a house (the pi bond) and the people living inside (the pi electrons).
Here's a breakdown:
Pi Bond
- Definition: A pi bond (π bond) is a covalent chemical bond where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital. This overlap occurs laterally (side-by-side) rather than head-on.
- Formation: Pi bonds are formed by the sideways overlap of p orbitals (or d orbitals in some cases). They always occur in addition to a sigma bond. A single bond is a sigma bond. A double bond is one sigma and one pi bond. A triple bond is one sigma and two pi bonds.
- Strength: Pi bonds are generally weaker than sigma bonds because the overlap of the p orbitals is not as effective as the head-on overlap in a sigma bond.
- Rotation: Molecules cannot freely rotate around a pi bond without breaking it, because rotation disrupts the parallel alignment of the p orbitals necessary for overlap.
- Example: In ethene (C₂H₄), the double bond between the two carbon atoms consists of one sigma bond and one pi bond.
Pi Electrons
- Definition: Pi electrons are the electrons that occupy the pi molecular orbitals of a pi bond.
- Location: They are located above and below the plane of the bonded atoms, in the region of space where the p orbitals overlap to form the pi bond. Because they are located further from the nucleus than sigma electrons, they are held less tightly and are therefore more reactive.
- Role: Pi electrons are responsible for many of the chemical properties of molecules containing pi bonds. They are more easily excited than sigma electrons, leading to absorption of UV and visible light, and they are also more susceptible to attack by electrophiles.
- Delocalization: In some molecules, pi electrons can be delocalized, meaning they are not confined to a single bond but are spread out over several atoms. This delocalization contributes to the stability of the molecule (e.g., in benzene).
- Example: In ethene (C₂H₄), there are two pi electrons forming the pi bond between the two carbon atoms.
Summary Table
| Feature | Pi Bond | Pi Electrons |
|---|---|---|
| Definition | A type of covalent chemical bond. | The electrons that occupy the pi molecular orbitals of a pi bond. |
| Nature | A physical bond between atoms. | Subatomic particles that are part of the bond. |
| Formation | Formed by sideways overlap of p orbitals. | Reside in the overlapping region of p orbitals that form the pi bond. |
| Function | Connects atoms; restricts rotation. | Determine reactivity and optical properties of molecules. |
In essence, a pi bond is the connection formed by the overlap of p-orbitals, and the pi electrons are the electrons within that connection, participating in the bond itself and influencing the molecule's properties.
