Hyperventilating about hypervalent atoms

This question came up after lecture and its one worth bringing to the o chem public: Should sulfuric acid be drawn with double bonds (the way I originally drew it in lecture) or with single bonds+formal charges (the way I subsequently drew it)?

There is more here than meets the eye.
If you look at Ch 1.6 – Exceptions to the Octet Rule, you will see that your book recommends the double bond formula. It states, “Elements of the third period and beyond have d orbitals that can be used for bonding. These elements can accommodate more than eight electrons in their valence shells.”

I don’t agree with that statement. To think about how
atoms share electrons, we should look at the energies of the orbitals
that the electrons might occupy. To share an electron, two
atoms must provide orbitals of similar energy. Otherwise, the electron
will hang out almost exclusively in the lower energy orbital.

As it happens, the 3s, 3p, and 3d orbitals on S and P (the elements we usually think of as “breaking” the octet rule) have very different energies. According to one model, the energies of these orbitals are -20, -11, and -8 eV for S, and -18.6, -14, and -7 eV for P. Compare the energy of a S 3d electron (-8 eV) or P 3d electron (-7 eV) with the energy of an O 2p electron (-14.8 eV) or a F 2p
electron (-18.1 eV). An electron that has to choose between S/P 3d
and O/F 2p would have a very strong preference for the low energy
orbitals provided by O and F. In fact, the electron might be so
partial to O/F that we shouldn’t say the electron is shared, we should
imagine an ionic bond instead.

Orbital energies have been known to chemists for decades, yet
many chemists persist in drawing hypervalent structures. Why? Perhaps because the drawings
are cleaner. Perhaps because certain wrong ideas have so much appeal (the earth is flat, the sun rises, tobacco smoke won’t harm me because I am young and beautiful) that logic and data no longer matter. If nothing else, this is a nice illustration that scientists are human and science is not always about getting the right answer.

Here’s what I expect you to do on Chem 201 assignments: When I ask
for a Lewis structure drawing, do not draw any exceptions to the octet rule. When any other kind of structural formula is needed, you can draw whatever other chemists draw.

Does this make sense? Is there a molecule that’s bugging you? Am I
forcing you to break a deeply engrained habit? Come see me. Or post a
comment – I think I have successfully activated them, but I won’t know
until you try to make a post.

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5 Responses to Hyperventilating about hypervalent atoms

  1. Patrick Fink says:

    So in conclusion, we should draw the sulfate ion with single bonds between sulfur and the oxygens, 3 lone pairs on each of Os, and a 2- formal charge on the S?

  2. Alan says:

    Patrick – you meant +2 formal charge on S (and -1 on each O), but otherwise – yes

  3. Patrick Fink says:

    That is of course what I meant. Too much time in the library and the counting starts to go.

  4. Patrick Fink says:

    PS. if you can, you should turn on the email notification of replies, so that we know if you’ve replied to our questions. -P

  5. Alan says:

    P. Thanks for the suggestion re: email notification. I think subscribing to the blog’s feed might accomplish that?

    FYI – The blog system+software are new to the college and new to me, so I’m not familiar with all of its features. Keep demanding the things you expect to find and I will pass your suggestions along. In this case, I don’t see where I “turn this feature on” but I will ask!

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