What is the role of Dipole moment to understand chemical bond?
In short, dipole moment can be used to understand chemical bonds and molecular shapes.
First, some background information: dipole moment is calculated as the product of a partial charge and distance.
Consider the molecule carbon dioxide (CO2). The electronegativity difference between carbon and oxygen causes an electron charge density toward the oxygen atom in each carbon-to-oxygen bond and gives rise to a bond moment.
However, because the two bond moments are equal in magnitude and opposite in direction, they cancel each other and lead to a resultant dipole moment of zero for the molecule:
O <- C -> O
The fact that CO2 is nonpolar is experimental proof that it is a linear molecule. Thus, we can determine the relationship between geometric shapes of molecules and the resultant dipole moments of molecules.
Another common example is water, H2O. H2O is a polar molecule. The electronegativity difference between H and O, and the bond moments combine to produce a resultant dipole moment of 1.84 D (debye). Unlike CO2, the H2O molecule cannot be linear, otherwise it would lead to a cancellation of bond moments. VESPR theory predicts that the H2O molecule is bent, and the observation that it is a polar molecule confirms the prediction.