Physics · Electric Charges And Fields · NEET
It is about the CENTRES of charge. Add up all positive charges and find their centre; add up all negative charges and find their centre. If these two centres sit at the SAME point, the molecule is non-polar (dipole moment p = 0). If they sit at DIFFERENT points, the molecule is polar and has a permanent dipole moment p = q x d, where d is the small separation between the centres. NEET tests the definition, not the chemistry of bonds.
Each C=O bond does pull charge, but CO2 is a straight line O=C=O. The two bond pulls point in exactly opposite directions and cancel. So the overall centre of negative charge lands right on top of the centre of positive charge (on the carbon). Net dipole moment = 0, so CO2 is non-polar. Water (H2O) is bent, not straight, so its pulls do NOT cancel and it stays polar. Shape decides it.
Water is bent at about 104.5 degrees, not a straight line. Oxygen pulls the shared electrons more strongly than the two hydrogen atoms, so the negative charge shifts toward oxygen and the positive charge toward the hydrogens. Because of the bent shape these shifts do not cancel, leaving a permanent dipole moment (about 6.1 x 10^-30 C m). That is why water is the classic NEET example of a polar molecule.
A permanent dipole exists on its own with no field, only in polar molecules. An INDUCED dipole appears only when you switch ON an external electric field: the field pushes positive charges one way and negative charges the other, so even a non-polar molecule (CO2, CH4) develops a temporary dipole moment. Remove the field and the induced dipole vanishes. NEET line to remember: non-polar molecules have zero PERMANENT moment but can get an INDUCED one in a field.
No. A polar molecule is electrically neutral overall (total charge = 0); it just has its + and - centres slightly apart, forming a dipole. An ion carries a real net charge. Do not confuse the two: a dipole is neutral but separated, an ion is charged. Polar molecules attract each other and align in a field, but they carry no net charge.
Polar molecules are the molecules:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Polar molecules: H2O (water) and HCl - they have a permanent dipole moment. Non-polar molecules: CO2 and CH4 - their charge centres coincide, so the permanent dipole moment is zero.
Yes. Although they have no permanent dipole, an external field pushes their positive and negative charges apart and creates an INDUCED dipole moment. This is why even CO2 and CH4 feel an electric field.
p = q x d, where q is the magnitude of the separated charge and d is the distance between the centres of positive and negative charge. It is a vector pointing from the negative centre to the positive centre. Unit: coulomb-metre (C m).
CO2 is non-polar. Even though each C=O bond is polar, the molecule is linear (O=C=O), so the two equal and opposite bond dipoles cancel exactly. The net dipole moment is zero.
A polar molecule already has a permanent dipole moment p, so when placed in a uniform field E it feels a torque tau = pE sin(theta) that tries to align it with the field. This is the next concept: torque on a dipole in a uniform electric field.
