dipole moment chapter 4 test [ENDORSED]

[Sammy Thatipelli 1B]

Why is it that for number 1 on test 4, there are no dipole moments for PO4^3- and PCl^5? I thought that there are dipole moments because the arrows point towards the negative oxygens (for PO4^3-) or negative chlorine molecules (PCl^5)?
Can someone explain how to know if there is a dipole moment or not?

[Brandon Fujii 1K]

PO₄^3- has a resonance structure which creates a uniform negative polar charge around the Phosphate (in a tetrahedral). Because the negative polar charge and strength of each Oxygen is equal, the charges cancel each other out and there is no dipole moment.

PCl⁵ is the same in the sense that each Chlorine's negative polar charge cancels the others out.

I hope that this helps

[Zane Mills 1E]

A bond dipole moment is created when you have polar bonds (such as in a molecule like Boron Trifluoride). There has to be a separation of negative charges from positive charges that act like vectors. It also depends on the distance between atoms. In the examples on the test, you can draw the lewis structures and they would help you determine whether or not a dipole moment is created between unlike charges in the molecule.

[Brigitte Phung 1F]

PO43- and PCl^5 do not have dipole moments because they have a symmetrical molecular shape (PO43- is a tetrahedral and PCl^5 is trigonal bipyramidal), which cancels out the dipole moments. Hope this helps.