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Bond Parameters

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Overview

On this page, we will discuss four important bond parameters:

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Bond Length

Bond length is defined as the equilibrium distance between the nuclei of two bonded atoms in a molecule. Each atom of the bonded pair contributes to the bond length. In case of a covalent bond, the contribution from each atom is called the covalent radius of that atom.

The bond length in a covalent molecule AB

Bond length

R = rA + rB (R is the bond length and rA and rB are the covalent radii of atoms A and B respectively.)

Bond Angle

The angle between the orbitals containing bonding electron pairs around the central atom in a molecule/ion is known as a bond angle.

Bond angle is expressed in degrees.

In water, the H-O-H bond angle is 104.5°

 Bond angle in water (H2O) molecule

Bond angle between H-C-H in CH4 molecule

 Bond angle in methane (CH4)

In order to give precise angles, minutes (') and seconds (") are used

Bond Enthalpy

The amount of energy required to break one mole of bonds of a particular type between two atoms in a gaseous state is known as bond enthalpy.

The unit of bond enthalpy is KJ mol-1.

H2 → H(g) + H(g);      ΔH = 435.8 KJ mol-1

Note : Since the bond enthalpy is associated with energy required to break a bond, it is evident that the larger the bond enthalpy, the stronger is the bond.

In case of polyatomic molecules, the enthalpy of each bond present is not the same. Therefore, in polyatomic molecules, mean or average bond enthalpy is used.

For example, the enthalpy required to break the two O-H bonds in H2O molecules is not the same.

H2O → H(g) + OH(g);      ΔH1 = 502 KJ mol-1

OH → H(g) + O(g);      ΔH2 = 427 KJ mol-1

Average bond enthalpy = (502+427)/2

= 464.5 KJ mol-1

Bond Order

The number of chemical bonds between two atoms in a molecule is known as bond order. In a covalent bond between two atoms, a bond order of one represents a single bond; a bond order of two represents a double bond; and a bond order of three represents a triple bond. A bond order of zero means there is no bond between atoms; thus, the molecule does not exist.

In molecules that exhibit resonance, the bond order may have a fractional value.

How to Find Bond Order

Diatomic molecules/ions: Steps involved to find bond order between two covalently bonded atoms:

  1. Draw the Lewis structure of the molecule.
  2. Calculate the number of bonds between the two atoms.
    Bond Bond Order
    Single bond 1
    Double bond 2
    Triple bond 3

    Some examples are given in the table below:

    Example
    Molecule/ion Lewis structure Bond Order
    H2 H:H 1
    O2 :Ö=Ö: 2
    N2 :N≡N: 3
    CO :C≡O: 3
    CN [:C≡N:] 3

Polyatomic molecules/ions: The following steps are involved:

  1. Draw the Lewis structure of the molecule.
  2. Count the total number of bonds in the molecule.
  3. Count the number of bond groups between individual atoms.
  4. Divide the number of bonds between atoms by the number of bond groups in the molecule.

Question : Determine the bond order for nitrate (NO3) ion.

Answer :

  1. The Lewis structure of nitrate ion is given below:
    Bond order of NO3- (nitrate) ion
  2. Total number of bonds in the molecule = 4
  3. Total number of bond groups between individual atoms = 3
  4. Hence, the bond order is 4/3 = 1.33

Important points regarding the bond order

  1. Isoelectronic species (species having the same number of electrons) have equal bond orders. For example, N2, NO+ and CO have 14 electrons; hence, all of them have the equal bond order of 3.
  2. Greater the bond order, greater is the stability of molecules — Since more energy is required to break more bonds.
  3. Greater the bond order, shorter is the bond length.
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