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My book says that from drawing the Lewis structure for the molecule below, you can conclude that it has dipole moment. Can someone please explain why?

structure of PF3

I understand that the dipole moment expresses the polarity of the bond. And that the dipole moment of a molecule is the sum of the dipole moments for each bond in the molecule.

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Fluorine is more electronegative than phosphorus.

The phosphorus is at the apex of a pyramid, the base of the pyramid being an equilateral triangle with a fluorine atom at each vertex. The F-P-F angles are 96 degrees.

Each F-P bond contributes to the net dipole moment, as a vector from the P to the F. The net dipole moment is the vector sum of the vectors along the three P-F bonds.

If all four atoms were in a plane (a trigonal planar geometry), there would be no net dipole moment. But because the geometry is pyramidal, the vector sum is not zero and there is a net dipole moment.

In some molecules lone pairs also contribute to the net dipole moment, in this particular case the lone pair of phosphorus is in a mostly s-like orbital, so in first approximation it can be neglected.

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