Bond Length in Chemistry

Bond Length Definition

• The average distance between the nuclei of two bonded atoms.
• Units of measurement: picometer (pm), angstrom (Å), and nanometer (nm).

Bond Length Examples

• Cl-Cl bond length: 198 pm
• C-C single bond length: 154 pm

Measurement of Bond Length

• Physical method: X-ray Diffraction Method
• This method involves measuring the diffraction angle of X-rays when they pass through a substance (ionic or covalent).
• The bond length can be calculated using Bragg's equation.

Factors Affecting Bond Length

• Connectivity: The type of atoms involved in the bond.
• Electronegativity difference: The difference in electronegativity between the two atoms.
• Number of bonds between two atoms: Single, double, and triple bonds.
• Percentage of s-character: The proportion of s-orbital character in the hybrid orbital involved in bonding.

Relationship Between Bond Length and Covalent Radius

• Additive Property: The bond length is roughly equal to the sum of the covalent radii of the two bonded atoms.
• Examples:
  • Cl-Cl bond length: 99 pm + 99 pm ≈ 198 pm
  • C-Cl bond length: 77 pm + 99 pm ≈ 176 pm

Effect of Number of Bonds

• Increase in the number of bonds between two atoms leads to a decrease in bond length.
• Example:
  • C-C single bond (154 pm) > C=C double bond (134 pm) > C≡C triple bond (120 pm)

Effect of Percentage of s-character

• Increased s-character leads to a shorter bond length.
• Example:
  • sp3 hybridization (33% s-character): Longer bond
  • sp2 hybridization (50% s-character): Shorter bond
  • sp hybridization (66% s-character): Shortest bond

Effect of Atomic Size

• As atomic size increases, bond length increases.
• Down a group in the periodic table: Atomic size increases, leading to longer bonds.
• Example: Na-Cl bond length > K-Cl bond length
• Across a period in the periodic table: Atomic size decreases, leading to shorter bonds.
• Example: C-Cl bond length < N-Cl bond length

Summary of Important Points

• Factors affecting bond length: Connectivity, electronegativity difference, number of bonds, percentage of s-character, and atomic size.
• Relationship between bond length and covalent radius: Additive property.
• Effect of increasing number of bonds: Decreases bond length.
• Effect of increasing s-character: Decreases bond length.
• Effect of increasing atomic size: Increases bond length.