Bond Parameters
Overview
On this page, we will discuss four important bond parameters:
- Bond length
- Bond angle
- Bond enthalpy
- Bond order
Let's get started.
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.
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 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
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:
- Draw the Lewis structure of the molecule.
- 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:
EXAMPLES 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:
- Draw the Lewis structure of the molecule.
- Count the total number of bonds in the molecule.
- Count the number of bond groups between individual atoms.
- 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 :
- The Lewis structure of nitrate ion is given below:
- Total number of bonds in the molecule = 4
- Total number of bond groups between individual atoms = 3
- Hence, the bond order is 4/3 = 1.33
Important points regarding the bond order
- 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.
- Greater the bond order, greater is the stability of molecules — Since more energy is required to break more bonds.
- Greater the bond order, shorter is the bond length.