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Bond Length

What is Bond Length

Atoms combine to form chemical bonds. Bond length is the average distance between the nuclei of two bonded atoms in a molecule. The number of electrons bonded to the two atoms determines the bond length [1-4].

Bond Length

Factors Affecting Bond Length [1,6]

  • The number of bonded electrons or bond order – The more the electrons, the shorter the bond is.

Order of bond length: single bond > double bond > triple bond

  • Bond strength and Bond disassociation energy – The stronger the bond, the shorter will be the bond length. In other words, if the bond strength increases, the bond length decreases.

How to Find Bond Length

There are two methods to find the bond length [1].

1. Using Electronegativity

Shoemaker and Stevenson proposed the following empirical formula to calculate the bond length.

dAB = rA + rB – 0.09 (xA – xB)

Where,

dAB: bond length between atoms A and B

rA: covalent radius of atom A

rB: covalent radius of atom B

xA: electronegativity of A

xB: electronegativity of B

2. Using Lewis structure

The bond length can also be determined from the Lewis structure of the molecule. Here are the steps.

  1. Draw the Lewis structure of the compound and identify the bond of interest.
  2. Look up the covalent radius of the two atoms from the chart.
  3. Calculate the sum of the two radii.

Examples of Bond Lengths

The following table lists and compares the bond lengths in picometer (1 pm = 10-12 m) between two atoms in all three types of covalent bonds [7].

CompoundBondBond length (pm)
Hydrogen (H2)H-H74
AlkaneC-C151
AlkeneC=C134
AlkyneC≡C120
 N-N142
 N=N120
Nitrogen (N2)N≡N108
 O-O129
Oxygen (O2)O=O114
 O≡O106
Carbon dioxide (CO2)O=C=O116
Water (H2O)H-O-H97
Ammonia (NH3)N-H101

Bond Length Trend

The trend for bond length is similar to that of the covalent radius. The reason is that the bond length is dependent on the covalent radius. It is the sum of the covalent radii of the bonded atoms. The covalent radius decreases from left to right across a period and increases from top to bottom down a group [1].

Bond Length and Bond Energy

The amount of energy required to break all covalent bonds of the same type in one mole of a compound in a gaseous state is called bond energy. The bond energy is inversely proportional to the bond length. In other words, shorter bond length has higher bond energy. The following table shows the bond length and bond energy of carbon-carbon bonds [8].

BondBond Length (pm)Bond Energy (kJ/mol)
C-C151347
C=C134614
C≡C120839

Bond Length and Resonance

Resonance is the delocalization of electrons in a conjugated system. The bond length of a single bond decreases since it gets some double bond character. The bond length of a double bond increases as it loses some of its double bond character. Both the single bond and double bond length change to an intermediate value. The resonance structures allow one to predict bond length [1].

For this reason, the carbon-carbon bond lengths in benzene are equal. The carbon-carbon bond alternates between C-C and C=C. The bond length between two carbon atoms is 139.9 pm, between C-C (151 pm) and C=C (134 pm).

References

  1. Chem.libretexts.org
  2. Courses.lumenlearning.com
  3. Thoughtco.com
  4. Chemistrytalk.org
  5. Chemistrysteps.com
  6. Mccord.cm.utexas.edu
  7. Chem.libretexts.org
  8. Chem.libretexts.org

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