Orientational PolarizationPublished on 24/2/2012 and last updated on 31/7/2018
A water molecule is bent structured. Here, oxygen atom has the covalent bond with two hydrogen atoms. Oxygen portion of the water molecule is slightly negative whereas hydrogen portions are slightly positive. These negative positive portions of the molecules form two dipole moments pointed from the center of oxygen atom to center of hydrogen atoms.
The angle between these two dipole moments is 105o. There would be a resultant of these two dipole moments. This resultant dipole moment is present in each of the water molecule even in the absence of any externally applied field. So, the water molecule has a permanent dipole moment. Nitrogen dioxide or similar types of molecules have same permanent dipole moment for same reason.
When an electric field is applied externally, the molecules with permanent dipole moment orient themselves according to the direction of applied electric field. This is because external electric field exerts a torque on the permanent dipole moment of each molecule. The process of orientation of permanent dipole moments along the axis of applied electric field is called orientational polarization.