1

We were taught in chemistry that:

  • Formal charges can be determined by drawing the Lewis dot structure, then subtracting the nonbonding electrons and half of the bonding electrons from the number of electrons ordinarily in that atom's valence.
  • Molecules tend to be in the state with the lowest formal charge on the central atom; in the event that there are multiple such forms, it will take on an in-between state called a resonance hybrid.
  • Three-dimensionally there's VSEPR theory which determines molecular geometry. We haven't gotten to how to calculate more complex angles just yet.

Consider ozone. There are two resonance structures for it: $\ce{O-O=O}$ and $\ce{O=O-O}$, both of which have equivalent formal charges: $(1)$ on the central atom, and $(-1)$ and $(0)$ on the axial atoms. This makes a resonance hybrid of $\ce{O\bond{~-}O\bond{~-}O}$, with bond order $\frac32$ for each bond.

Why isn't a third structure accounted for? Consider ozone structured like a triangle, with a single bond attaching each oxygen to the other two, and four unbound electrons on each oxygen:

Triangular ozone

  • The formal charge for every oxygen would be $(0)$.
  • The bond order would be $1$ for every bond; if merged as a resonance structure with the above two, it would create a structure with a bond order of $\frac43$ between the top oxygen and each side oxygen, and a bond order of $\frac13$ between the bottom two oxygens.

Yet the empirical evidence supports a bond order of $\frac32$ as a bent molecule, not a bond order of $\frac43$ as a triangular molecule. Why not?

andselisk
  • 37,604
  • 14
  • 131
  • 217
DonielF
  • 507
  • 4
  • 13
  • Also, if anyone can help out with mhchem formatting to write out my triangular ozone rather than my comparatively ugly computer sketch, that would be great. – DonielF Dec 12 '19 at 18:45
  • Does Can ozone have a triangular structure? answer your question? – andselisk Dec 12 '19 at 18:52
  • As for the illustration, I'm afraid this isn't a task for mhchem. An external tool is required to draw the structure, which is then added as a raster image. – andselisk Dec 12 '19 at 18:54
  • @andselisk That’s a shame that mhchem doesn’t support that. Anyway, that link does seem to ask the same question, but the chemistry in the answer is a bit above my pay grade. You can close as a duplicate. – DonielF Dec 12 '19 at 18:55
  • Well, you can re-formulate the question focusing on the aspects you don't understand and put a link to the current duplicate. – andselisk Dec 12 '19 at 18:57
  • In the meantime I've redrawn the image with the font size used for the text. If it looks too tiny for you, here is a doubled version. – andselisk Dec 12 '19 at 19:12
  • @andselisk Thanks for the image - where can I find software like that to make such an image myself in the future? For the time being I think it’s probably better to close this as a dupe and once I’ve learned a bit more chemistry I can come back and revisit the topic. – DonielF Dec 12 '19 at 19:20
  • @DonielF No prob. This one is made with chemfig, I also occasionally use MarvinSketch. For a more complete and less opinionated list check out software. Closing the question as a dupe then. – andselisk Dec 12 '19 at 19:27

0 Answers0