Short Answer

Triple bonds are shorter than double bonds because they involve more shared electron pairs, which pull the bonded atoms closer together. A triple bond has one sigma bond and two pi bonds, while a double bond has only one sigma bond and one pi bond. The increased electron density in a triple bond creates a stronger attraction between the atoms.

As a result, triple bonds have greater bond strength and tighter bonding, causing the atoms to stay closer. This is why molecules like nitrogen gas (N₂) have very short bond lengths compared to molecules with double bonds, such as oxygen (O₂).

Detailed Explanation :

Why Triple Bonds Are Shorter Than Double Bonds

Bond length refers to the distance between the nuclei of two bonded atoms. The length of a bond depends on the attractive forces holding the atoms together. Stronger bonding forces pull atoms more tightly, resulting in a shorter bond length. Triple bonds, which involve three shared pairs of electrons, create greater pulling force than double bonds, which only involve two shared pairs. This fundamental difference in electron sharing explains why triple bonds are shorter.

Understanding this concept is important in chemistry because bond length affects molecular structure, stability, reactivity, and even physical properties like boiling and melting points.

1. Number of Shared Electron Pairs

The most direct reason for the shorter length is the number of electron pairs shared between two atoms.

• Double bond = 2 shared pairs (1 sigma + 1 pi)
• Triple bond = 3 shared pairs (1 sigma + 2 pi)

More shared electron pairs mean:

• Higher electron density between the nuclei
• Stronger attraction between atoms
• Greater pulling force
• Shorter distance between atomic centers

Thus, the extra pi bond in a triple bond significantly strengthens and shortens the bond.

2. Sigma and Pi Bonding Differences

Both double and triple bonds include a sigma (σ) bond, which forms by head-on overlap. However:

• A double bond has one pi bond
• A triple bond has two pi bonds

Pi bonds form by sideways overlap of p orbitals, increasing electron density above and below the internuclear axis. When two pi bonds are present:

• Electron distribution becomes even more concentrated
• Atoms are pulled closer
• Bond length decreases

This combination of one sigma and two pi bonds makes triple bonds extremely strong.

3. Bond Strength and Bond Length Relationship

Bond strength and bond length are closely related:

Stronger bond = shorter bond

• Triple bonds are the strongest
• Double bonds are moderately strong
• Single bonds are the weakest

Examples:

Bond Type	Bond Length (Approx.)
C≡C (triple)	120 pm
C=C (double)	135 pm
C–C (single)	154 pm

This pattern clearly shows how increasing the bond order decreases the bond length.

4. Greater s-Character in Hybrid Orbitals

Hybridization also affects bond length:

• Triple-bonded carbon uses sp hybridization
• Double-bonded carbon uses sp² hybridization

In sp hybridization:

• The s-orbital contributes 50%
• In sp², s-character is only 33%

Greater s-character pulls electrons closer to the nucleus, resulting in:

• Stronger bonding
• Shorter bond length

Thus, sp hybridized atoms form tighter and shorter connections.

5. Stronger Overlap of Orbitals

Triple bonds involve more overlapping orbitals than double bonds. When orbitals overlap more effectively:

• Bond becomes stronger
• Nuclei are pulled closer together
• Bond length decreases

This enhanced overlap is especially seen in molecules like N₂, where the 1σ + 2π bond system creates one of the shortest and strongest bonds in chemistry.

6. Real Examples Showing the Difference

Nitrogen gas (N₂):

• Triple bond
• Very short and strong
• Bond length: ~110 pm

Oxygen gas (O₂):

• Double bond
• Longer and weaker compared to N₂
• Bond length: ~121 pm

These values clearly show how triple bonds shorten the distance between atoms.

7. How Short Bond Length Affects Reactivity

Shorter bonds are generally:

• Stronger
• Harder to break
• Less reactive in some conditions

For example:

• Triple bonds in N₂ make it very stable and unreactive
• Double bonds in alkenes are more reactive because the bond is weaker

Thus, the shorter bond length contributes to the overall chemical behavior of the molecule.

Conclusion

Triple bonds are shorter than double bonds because they involve three shared electron pairs instead of two. The extra pi bond increases electron density and strengthens the attraction between the atoms, pulling them closer. Hybridization, stronger orbital overlap, and greater s-character all contribute to shorter bond lengths in triple-bonded molecules. Understanding this relationship helps explain many structural and reactivity patterns in chemistry.