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Dipole moment exists in polar molecules. In polar molecules, one atom has a partial positive charge, and the other atom has a partial negative charge. Thus, the dipole moment is a way of representing charge separation in a polar molecule. The direction of the dipole moment is from the positive end to the negative end.

Let's take an example of a polar molecule, say H-Cl. The hydrogen atom (H) has a partial positive charge, and Chlorine (Cl) has a partial negative charge. So it has a dipole moment, and its direction will be from hydrogen atom to chlorine atom.

$\underset{↦}{H−Cl}$

In this, all the three oxygen atoms are in resonance, and two negative charges are equally contributed to the three oxygen atoms to form a resonance hybrid.

Structure of carbonate ion

There is a polarity difference between the carbon atom and oxygen atoms in carbonate ions. According to VSEPR theory, the shape of the carbonate ion is trigonal planar, and the molecule is symmetrical because the carbonate ion forms a resonance hybrid, i.e., all the three resonance forms will contribute equally.

Dipole moment in carbonate ion

Also, each bond has polarity in it, but when we draw the directions of each dipole as they are opposite to each other, they will cancel out. So there is no net dipole moment, or we can say that the carbonate ion has zero dipole moment.