Bond Parameters:
1. Bond length:
Bond length is defined as the equilibrium distance between the nuclei of two bonded atoms in a molecule. Bond lengths are measured by spectroscopic, X-ray diffraction and electron-diffraction techniques. Bond length is usually expressed in Angstrom units (0Α ) or picometers (pm).
Bond length depends upon the size of the atoms and nature of bonds.Bond length increases with the increase in the size of the atoms, e.g., bond length between hydrogen and chlorine atom in HCl molecule is 127 pm whereas bond length between carbon and chlorine atoms in C-Cl bond is 177 pm.
Also, bond length decreases with the multicity of bonds.
Single bond > Double bond > Triple bond
For example, bond length of C-C bond is 154 pm whereas that of C=C bond is 133 pm.
Bond angle : It is defined as the angle between the orbitals containing bonding electron pairs around the central atom in a molecule/complex ion or angle between two adjacent bonds at an atom in a molecule made up of three or more atoms is known as bond angle.
Bond Angle:
Bond angle is expressed in degree/minute/seconds. For example, H-C-H bond angle in CH4 molecule is 109028’. Similarly , F-B-F bond angle in BF3 is 1200 and H-N-H bond angle in NH3 molecule is 1070. Bond angle is experimentally determined by spectroscopic methods. It tells us regarding the distribution of orbitals around the central atom in a molecule/complex ion and hence it helps us in determining its shape.
Bond enthalpy:
Bond enthalpy is defined as the amount of energy required to break one mole of bonds of a particular type between two atoms in gaseous state. The unit of bond enthalpy is kJ mol-1, measured at 298 K.
Bond breaking is an endothermic process, and the bond enthalpy involved is given a positive sign.The magnitude of bond enthalpy increases with the multiplicity of bond even though the atoms involved in the bond formation are same, because the attractive force between nuclei and electrons also increases. For example, bond enthalpy of C-C bond is 348 kJ/mol but that of C = C bond is 619 kJ mol-1. The bond enthalpy for molecules containing multiple bonds, for example O2 and N2 are as under:
More the bond dissociation enthalpy, stronger will be the bond in the molecule.
Bond Length in Polyatomic molecules:
The measurement of bond length is more complicated. For example, consider methane, CH4. It contains four C-H bonds, and it seems reasonable that they should all have the same bond enthalpy. However, if you took methane to piece one hydrogen at a time, it needs a different amount of energy to break each of the C-H bonds. When you break a hydrogen off the carbon, the environment of those left behind changes. And the strength of a bond also changes. Here the bond enthalpy quoted is an average value.
In the case of methane, you can work out how much energy is needed to break a mole of methane gas into gaseous carbon and hydrogen atoms. That comes to +1662 kJ and involves breaking 4 moles of C-H bonds. The average bond energy is therefore +1662/4 kJ, which is +415.5 kJ per mol of bonds.
4. Bond order:
As per the Lewis description of covalent bond, bond order is the number of chemical bonds between a pair of atoms in a molecule. The bond order, for example in diatomic molecule like H2 *with single shared electron pair), in O2 (with two shred electron pairs) and in N2 (with three shared electron pairs) is 1, 2, 3 respectively.Bond order gives an indication to the stability and strength of a bond. Bond order , bond enthalpy and bond length are related to each other as
With increase in bond order, bond enthalpy increases and bond length decreases.