intermolecular forces/polarity

[Katherine Wu 1H]

Explain the difference in the boiling points of AsF3 (63 celsius) and AsF5 (-53 celsius).
The answer says that it is clear that AsF3 is a polar molecule while AsF5 is not from the Lewis structures. How do you tell whether a structure is polar or not?

[Benjamin Feng 1B]

You have to draw out the Lewis structure to see if there is a dipole moment. In AsF3, the molecule is symmetric with no lone pairs so even if there is a polar bond between As and F, they are pulling electrons all in different directions so the net molecule has no polarity. On the other hand, AsF5 has lone electron pairs, which create a dipole moment and thus make the molecule polar. This means AsF5 exhibits dipole dipole attractions and has a higher boiling point.

[sarahsalama2E]

Higher attraction (aka dipole moment) means that it takes more energy to disassociate the bond, which results in a higher melting or boiling point. Like what was mentioned before, you must draw the lewis structure to see if the bonds cancel out, because the particular bonds could be polar but the molecule as a whole could be NON polar.