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Resonance structures are necessary when you can draw the same arrangement of atoms with different arrangements of e-. This occurs on account of the number of double bonds and lone pairs on each atom (you can find the best arrangement using formal charge, which we learned about last lecture).
Last edited by Andrew Pfeiffer 2E on Sun Oct 27, 2019 10:40 pm, edited 1 time in total.
Resonance structures are necessary because the e- is not just occupying that one formation. The electrons are actually in a position that is an average of the possible structures so saying that one is correct is not true. We draw all of them because it shows that there is more than one possibility of how the electrons can be arranged.
From what I understand of resonance, those alternative structures Dr. Lavelle shew us in his lecture are contributing resonance structures of the molecule because all of these structures are plausible states of the molecule but with different formal charges and electron distributions. However, experimental data shows that the real structure of the molecule is a combination of all its contributing resonance structures due to de-localization of electrons in the molecule. For example, NO3- has two single bonds and one double bond. It has three contributing resonance structures as a single nitrogen atom can have double bond with any of the three oxygen atoms. Since none of the three structures is a true representation of the molecule, the real structure of NO3- is a combination of all three structures, and the nitrogen atom forms three identical bonds with three oxygen atoms with bond length between a double bond and a single bond. Hope this can help!
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