rm_27
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Hey guys,

I was doing a question and it said to predict the shape of molecules BF3 and PF3, which I did. I found there was 3 bonded pair of electrons and 0 lone pairs for BF3 (making it have a trigonal planar shape), and 3 bonded electron pairs with 1 lone pair for PF3 (Pyramidal).

It then said to explain why BF3 is non polar and PF3 is polar.

The answer was: BF3 has polar bonds but the molecule has a symmetrical trigonal planar shape. Over the whole molecule, the dipoles cancel out.

- How can a molecule have polar bonds but be symmetrical at the same time?
- How does the dipole cancel out?
- Wouldn't the fact that PF3 has a lone pair contribute to it being polar?

Thank you!
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Pigster
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Compare the molecules to three equal forces acting on an object:

In BF3 the three forces will cancel out and the object will not move. The molecule is non-polar.

In PF3, the three forces all pull away from each other, but also downwards. The object moves downwards. The molecule is polar.
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