Pi Bond
Definition and meaning of Pi Bond in chemistry.
A pi bond is a type of covalent chemical bond formed by the sideways overlap of atomic orbitals. It represents the second and third bonds found in double and triple covalent connections between atoms.
In more detail
A pi bond is a specific type of covalent bond formed when two parallel atomic orbitals overlap sideways above and below the direct line connecting two atomic nuclei. In chemistry, the strongest and most fundamental connection between two atoms is the sigma bond, which forms from the direct, head-on overlap of orbitals.
However, when atoms need to share more than one pair of electrons, they cannot simply cram another bond directly between the nuclei because the electrons would repel each other. Instead, the extra shared electrons are forced to occupy the space above and below the main sigma bond, creating a pi bond.
Pi bonds are exclusively found in molecules that contain double or triple bonds. A standard double bond is always composed of exactly one strong sigma bond and one weaker pi bond. A triple bond consists of one central sigma bond surrounded by two pi bonds.
Because the overlap of the electron clouds in a pi bond is sideways and somewhat dispersed, rather than concentrated tightly between the nuclei, a single pi bond is fundamentally weaker than a single sigma bond. However, the combination of the two bonds makes a double bond much stronger overall than a single bond.
One of the most important chemical consequences of a pi bond is that it completely prevents the bonded atoms from freely rotating. In a single sigma bond, atoms can spin like wheels on an axle. But because the pi bond locks the atoms together from above and below, twisting the molecule would physically break the pi bond.
This rigid locking mechanism creates geometric isomers in organic chemistry, forcing molecules into permanent 'cis' or 'trans' shapes that dictate how they interact biologically.
Key facts
| Field | General Chemistry |
|---|---|
| Formation | Sideways overlap of parallel p-orbitals |
| Location | Electron density is above and below the internuclear axis |
| Strength | Weaker than a single head-on sigma bond |
| Presence | Found only in double and triple bonds |
| Physical Effect | Prevents free rotation of the bonded atoms |
In a molecule of ethene (C2H4), the two carbon atoms are locked together by a double bond, which consists of one direct sigma bond and one sideways overlapping pi bond.
Frequently asked questions
Can a pi bond exist without a sigma bond?
No, a strong central sigma bond must always form first to pull the atoms close enough together for the sideways pi bond to overlap.
Why is a pi bond weaker than a sigma bond?
The sideways overlap is much less efficient and less concentrated than the direct, head-on overlap of a sigma bond.
How many electrons are shared in a pi bond?
Like almost all standard covalent bonds, a single pi bond represents the sharing of exactly two electrons.