Chemistry BOND LENGTH AND BOND ENERGY

Bond Energy :

It has already been pointed out that the formation of a bond occurs as a result of decrease of energy. Therefore, same amount of energy is required to break the bond between the two atoms. For example, the energy released during the formation of bonds between the gaseous hydrogen atoms to form one mole of hydrogen moleculs is `433` `kJ` `mol^(-1)`. This energy involved in making or breaking of bonds is referred to as bond energy. Thus, bond energy may be defined as the amount of energy required to break one mole of bonds of same kind so as to separate the bonded atoms in the gaseous state.

The magnitude of bond energy reflects the strength of the bond. Its magnitude depends upon the following factors :

(i) `text(Size of the participating atoms)` : Larger the size of the atoms involved in bond formation, lesser is the extent of overlapping and consequently, smaller is the value of bond energy. For example, bond energy of `Cl-Cl` bond is `237` `kJ` `mol^(-1)` whereas that of `H-H` bond is `433` `kJ` `mol^(-1)`.

(ii) `text(Multiplicity of bonds)` : The magnitude of bond energy increases with the multiplicity of bonds even though the atoms involved in the bond formation are same. It is because of the fact that with the multiplicity of bonds the number of shared electrons between the atoms increases. As a result, the attractive force between nuclei and electrons also increases and consequently, the magnitude of bond energy increases. For example, bond energy of `C-C` bond is `348` `kJ` `mol^(-1)` but that of `C=C` bond is `619` `kJ` `mol^(-1)`. The average bond energies of some bonds are given in Table below.

(iii) `text(Number of lone pairs of electrons)` : Greater the number of lone pair of electrons present on the bonded atoms, greater is the repulsive interactions between them and smaller is the bond energy.

Bond Length :

It has already been discussed that two bonded atoms in a molecule remain held up at a certain distance from each other. They cannot approach too close because it leads to repulsive interactions and potential energy of system increases. This minimum distance between the bonded atoms is referred to as bond length. Thus, bond length may be defined as the average distance between the centres of nuclei of the two bonded atoms in a molecule . Bond length is usually expressed in Angstrom units `(A^o)` or picometers (pm) and it can be determined experimentally by `X`-ray diffraction and other spectroscopic techniques.

(i) `text(Bond length increases with the increase in the size of the atoms)` : Bond length between hydrogen and chlorine atoms in `HCl`
molecule is `127` pm whereas bond length between carbon and chlorine atoms is `C-Cl` bond `177` pm.

(ii) `text(Bond length decreases with the multiplicity of bonds)` : It is because of the fact that larger the number of electrons shared by the two atoms greater will be attractive force between electrons and the nuclei and consequently, lesser is the bond length. For example, bond length of `C-C` bond is `154` pm whereas that of `C=C` bond is `134` pm. Bond lengths of some common bonds are given in Table below.

Bond Angle :

We know that covalent bonds are formed by overlapping of atomic orbitals. Due to directional character of atomic orbitals, the covalent bonds in a molecule are oriented in specified directions. The bond angle is defined as the average angle between the lines representing the orbitals containing the bonding electrons.

Bond angle is expressed in degree/minute/seconds. For example, `H-C-H` bond angle in `CH_4` molecule is `109^o 28^'`. Similarly, `F-B-F` bond angle in `BF_3` is `120^o` and `H-N-H` bond angle in `NH_3` molecule is `107^o`. The bond angles in `CH_4`, `NH_3`, `H_2O` and molecules are shown below in fig.1.

`text(Following factors can affect bond angle)` :

(i) `text(Hybridisation of central atom)` :

`sp` `180^o`
`sp^2` `120^o`
`sp^3` `109.5^o`
`sp^3d` `90^o, 120^o, 180^o`
`sp^3d^2` `90^o, 180^o`
`sp^3d^3` `72^o, 90^o, 144^o, 180^o`-

(ii) `text(Presence of lone pair)` : lp -bp repulsion is more than bp-bp. See fig.2.

(iii) `text(Electronegativity of central atom)` : Due to more electronegativity of central atom bond pair get shifted toward it and distance between them decrease thus further bp-bp repulsion increases. See fig.3.

(iv) `text(Multiple bonds)` : Due to more electron density for double bond than that of single bond, repulsion due to double bond is more than single bond. See fig.4.