Bond length | Measurement and variation of bond length

The distance between the nuclei of two covalent bonded atoms is called the bond length.

Example:
In a C–H bond, the bond length is about 1.09 Å.

Measurement of Bond Length:

The bond length are experimentally determined by physical techniques, X-ray diffraction or spectral studies are important method. The Covalent radii for different elements are almost additive in nature. The single bond covalent radius of carbon is 77 pm which is half of the C–C bond length (154 pm). Similarly, the covalent radius of Cl is 99 pm. it is one half of the Cl–Cl bond length (198 pm) some selected bond lengths are given.

Compound	Hybridization	Bond	Bond length (pm)
BF₃ (sp² hybridized)	Sp2	B–F	130
BCl₃ (sp² hybridized)	Sp2	B–Cl	175
SiH₄ (sp³ hybridized)	Sp3	Si–H	148
SiF₄ (sp³ hybridized)	Sp3	Si–F	155
C₂H₆ (Ethane) (sp³ hybridized)	Sp3	C–C	154
C₂H₄ (Ethene) (sp² hybridized)	Sp2	C=C	133
C₂H₂ (Ethyne) (sp hybridized)	sp	C≡C	120
(CH₃)₂C=O (Acetone) (sp² hybridized)	Sp2	C=O	122

Factors Affecting Bond Length:

1. Electronegativity difference:
   As electronegativity difference between the bonded atoms increase, the bonds become shortened and departure from additivity of bond lengths takes place.
   Example:
   Si-F bond length in SiF₄ is found to be 159 pm, whereas the addition of their covalent radii (Si = 117 pm and F = 64 pm) gives Si-F bond length to be equal to 181 pm. The calculated values are almost always higher due to electronegativity differences. The ionic character results in shortening of the bond length due to force of attraction between the polar ends.
2. Type of Hybridization:
   Hybridization involved also explains the shortening of bonds due to the predominant participation of s orbitals. Since the 2_s orbital of carbon has smaller average radius than the 2_p orbitals, it would be expected that greater the s character in the hybrid orbitals used, the shorter will be the bond distance. Thus the C-C bond lengths are 154, 133 and 120 pm for ethane, ethene and ethene, respectively. The s orbital contribution increases from sp³ to sp. Further x-bonding also reduces the internuclear bond distance.

Variation of Bond lengths in a Group.

The bond of length increases as we move from top to bottom in a group of the periodic table. Thus Si-Si bond length is more than C-C bond length and P-P bond length is much more than N-N bond length. As the atomic radii increases in a group (N to P or C to Si), the effect of the effective nuclear charge decreases on electron. Thus bond length will increase.

Variation of Bond length in a period :

In the periodic table, shortening of bond length occur from left to right in a period. This can be attributed to the pull by nuclear charge with same value of ‘n’. therefore, C-C bond length is greater than N-N bond length.

How to calculate bond length

Bond length is calculated by adding the covalent radii of the two bonded atoms.

Bond length=Covalent radius of atom A + Covalent radius of atom B

Example:
C–H bond = 0.77 Å + 0.37 Å = 1.14 Å