Bond length : What is bond length ?


The bond length of the covalent bond is the nuclear seperation distance where the molecule is most stable.Or in simple words, bond length is the distance between the nuclei in a bond .The H-H bond length in moleular hydrogen is 74 pm.At this distance,attractive interactions are maximied relative to repulsive interreactions.

The bond length are determined experimentally using x-ray diffraction or analysis of molecular spectra.

The table given above for hte most common chemical bonds average bond length displays several trends.

The one trend is that bonds become longer as the radii of the atoms become larger.This can be explained by the fact that bonding involves valence orbitals and it is filled valence orbitals that determine size of an atom.As atomic size increases with principle quantum number,bond lengths vary predictably with the n-value of the valence orbitals.

Nuclear charge also effect the bond length.As atomi size decreases from left to right across a row of periodic table because of progressive rise in nuclear charge.Smaller orbitals form shorter bonds,so Cl-Cl bonds are shorter than S-S bonds and S-S bonds are shorter than P-P bonds. 

The bond length show one feature also that is multiple bond is shorter than the corresponding single bond between two atoms: This means  the triple bonds are the shortest of all bonds amoung second-row elements.

To summarize the following factors influence the bond lengths:

  1. The smaller the atoms,the shorter the bond.
  2. The higher the bond multiplicity,the shorter the bond.
  3. The higher the effective nuclear charges of the bonded atoms,the shorter the bond.
  4. The larger the electronegativity difference of the bonded atoms,the shorter the bond.

Lets see a solved example:

Sample probelm: What factors accounts for each of the following differences in the bond length?

a. I2 has longer bond than Br2

b. C-N bonds are shorter than C-C bonds.

c. H-C bonds are shorter than Ctriple bond O .


a. Step 1: Iodine is just below Br in the periodic table,

  Step 2: This will result in the valence orbitals of iodine larger than the valence orbitals of Br.

  Step 3: Thus, I2 is longer than Br2 bond because iodine has larger atomic radius.

b. Step 1: Carbon and nitrogen are second-row elements.

    Step 2 : Nitrogen has higher nuclear charge than carbon,however,so nitrogen has the smaller radius.This makes C-N bonds shorter than C-C bonds.

  Step 3 : In addition,a C-N bond is polar,which conributes to the shortening of the C-N bond.

c   Step 1:  Here, we are comparing bond in which the atomic radii and the amount of multiple bonding influence bond length.

   Step 2 : The experimental fact that H-C bonds are shorter than triple bond between c and oxygen indicates that size of the hydrogen orbital is a more important factor tha the presence of multiple bonding.