[KYPowerlifter]

I am curious as to how one would know that POCl3 is exhibits resonance? That is to say, how do you know that it's simply not a single bonded molecule with 32 valence electrons, with Phosphorus having a formal charge of 1+? I understand that it is a resonance molecule with a 3 single bonds and a double bond to Oxygen also; with a formal charge on the P of 0. Is it just the fact that all period 3 and beyond molecules, if possible, exhibit resonance via their "d" orbitals, in combination with the fact that the most stable lewis structures contain atoms that have the lowest charge possible? I'm having trouble understanding how to realize that the double variety is a possibility and that it is also accepted as "best", as well as assigning a formal charge of 0...

Thanks

[macman104]

You mean the resonance between these two structures?  Are you asking why we know the second one is the predominant form, and not the first one?

[KYPowerlifter]

macman 104:

Yes, that is what I mean.

Frankly, I do not understand why it is merely not expressed in the first form only, thus having a formal charge of 1+. Unless, it is because one always expresses 3rd period and beyond elements with more than an octet, when possible (along with the notion that the most stable lewis structures contain atoms with the lowest possible charge: 0 being lower than 1)?

Perhaps this is just verified by empirical observation and something that has to be remembered?

Appreciate the reply...

[macman104]

The reason is because a charge separated resonance structure has a higher energy and is less stable than a non-charge separated structure.  There is a release of energy when the double bond forms because the positive and negative charge naturally want to cancel.

The 3rd period structures can fix this charge separation by utilizing their d orbitals, so...they do.

[KYPowerlifter]

Concise, cogent, clear...

Much obliged!