The bond energy is defined as the minimum amount of energy required to break one mole of a particular bond in molecules in their gaseous state.

In the case of covalent bond formed between atoms having different electronegativities, the atom with higher electro negativity will have greater tendency to attract the shared pair of electrons more towards itself than the other atom. As a result the cloud of shared electron
pair gets distorted. Let us consider the covalent bond between hydrogen and fluorine in hydrogen fluoride. The electro negativities ofhydrogen and fluorine on Pauling's scale are 2.1 and 4 respectively. It means that fluorine attracts the shared pair of electrons approximately twice as much as the hydrogen which leads to partial negative charge on fluorine and partial positive charge on hydragen. Hence, the H-F bond is said to be polar covalent bond.

Molecules having polar bonds will not necessarily have a dipole moment. For example, the linear form of carbon dioxide haszero dipole moment, even though it has two polar bonds. In CO₂, the dipole moments of two polar bonds (CO) are equal in magnitude but have opposite direction. Hence, the net dipole moment of the CO₂ is,
\(\mu=\mu_{1}+\mu_{2}=\mu_{1}+(-\mu_{1})=0.\)

In this case \(\mu=\overrightarrow { { \mu }_{ 1 } } +\overrightarrow { { \mu }_{ 2 } } \quad \)
\(=\overrightarrow { { \mu }_{ 1 } } +\overrightarrow { ({ \mu }_{ 1 }) } =0\)
Incase of water net dipole moment is the vector sum of \(\mu_{1}+\mu_{2}\) as shown.

Dipole moment in water is found to be 1.85 D

a) octahedral - sp³d²
b) tetrahedral - sp³
c) square planer - dsp²

'When we write Lewis structures for a molecule, more than one valid Lewis structures are possible in certain cases. For exampte let us consider the Lewis structure of carbbnate ion [CO₃]^2- . The skeletal structure of carbonate ion (The oxygen atoms are denoted as O_A, O_B & O_c

2. Total number of valence electrons = [1 x 4(carbon)] + [3 x 6 (oxygen)] + [2 (charge)] = 24 electrons.
3. Distribution of these valence electrons gives us the following structure.

4. Complete the octet for carbon by moving a lone pair from one of the oxygens (O_A) and write the charge of the ion (2-) on the upper right side.

5. In this case, we can draw two additional Lewis structures by moving the lone pairs from the other two oxygens (O_B & O_c) thus creating three similar structures as shown below in which the relative position of the atoms are same. They only differ in the position of bonding and lone pair of electrons. Such stnictures are called resonance structures (canonical structures) and this phenomenon is called resonance.

6. It is evident from the experimental results that all carbon-oxygen bonds in carbonate ion are equivalent. The actual structure of the molecules is said to be the resonance hybrid, an average of these three resonance forms. It is important to note that carbonate ion does not change from one structure to another and vice versa. It is not possible to picturise the resonance hybrid by drawing a single Lewis structure. However, the following structure gives a qualitative idea about the correct structure

7. It is found that the energy of the resonance hybrid (structure 4) is lower than that of all possible canonical structures (Structure 1, 2 & 3). The difference in energy between structure 1 or 2 or 1 3, (most stable canonical structure) and structure 4 (resonance hybrid) is called resonance energy.