Number of Resonance Strucutres

[Rebekah Kaufman 1L]

Is there a way to know how many possible resonance structures a molecule will have? Or is drawing them the only way to figure that out?

[DomMaiorca_1I]

Finding the formal charge is usually a good indicator for how many resonance structures you'll have. For example, if you calculate the formal charges in PO4^3-, you find that P= +1 and 0= -1. Knowing the formal charges can cancel, and also that there are four times as many O as P, you know you'll have four total resonance structures before you start drawing them.

Outside of this strategy, I'm unaware of any quicker tricks that'll tell you exactly how many resonance structures you'll have. Luckily this way isn't too difficult, just a bit time consuming.

[Jessica Urzua-1H]

I found this website that describes how to identify resonance structures from Lewis structures and what they should consist of. I found it very helpful.

http://shimizu-uofsc.net/orgo/Chem_333/1c.i.html

[Andre-1H]

Even though drawing them can be painstaking, I'm pretty sure it's a solid way to figure it out. Maybe it wouldn't necessarily be the best way to do it on a test but I think getting comfortable with drawing structures is good practice to save time on tests.