Are these 3 molecular structures of ammonium bicarbonate the same?







  • 1
    $\begingroup$ As far as representations for molecular structures go, B seems wrong to me, as there is no oxygen nitrogen bond in the compound. $\endgroup$ May 25 '15 at 8:58
  • 1
    $\begingroup$ Agreeing with @Martin-マーチン, what are the sources of these images, just out of curiousity $\endgroup$
    – user15489
    May 25 '15 at 9:04
  • $\begingroup$ This is from a chemicals company's website. mpbio.com/product.php?pid=02150107&country=96 $\endgroup$ May 25 '15 at 9:10
  • $\begingroup$ The site says "MP Biomedicals is a global corporation, with headquarters in Santa Ana, California, USA and regional offices in Europe, Asia Pacific and Australia. " $\endgroup$ May 25 '15 at 9:11
  • 3
    $\begingroup$ Structure C is even worse! Not only is there a non-existent $\ce{N-O}$ covalent bond, but such a bond would exceed the octet on the nitrogen atom! $\endgroup$ May 25 '15 at 11:00

Like Punarbasu Roy already stated, the depicted structures are certainly not the same. To be frank, only one of them (A) is correct, one of them is not completely wrong (B - but only because it is incomplete), and one is completely, utterly wrong (C).

In lieu of an actual crystal structure of ammonium hydrogencarbonate,[1] I'll refer to the structure of the parent compound ammonium carbonate monohydrate to make my point.[2] It is actually quite difficult to assign one correct Lewis structure. In its solid form ammonium (bi)carbonate is a salt, in which ammonium and (bi)carbonate ions are interacting via ionic bonds in a lattice. In solution this certainly is broken up and we will find separate ions, according to the following equilibria: \begin{align} \ce{(H4N)HCO3~(s) &~->C[(H2O)] NH4+ ~(aq) + HCO3- ~(aq)}\\ \ce{NH4+ ~(aq) + H2O &~<=> H3+O~(aq) + NH3~(aq)}\\ \ce{HCO3- ~(aq) + H2O &~<=> H2CO3 ~(aq) + {}^{-}OH~(aq)}\\ \ce{HCO3- ~(aq) + H2O &~<=> CO3^2- ~(aq) + H3+O~(aq)}\\ \end{align} The same equilibria will establish for ammonium carbonate with different concentrations.

In the crystal structure of ammonium carbonate monohydrate you can see the different molecular entities (ions) held together via hydrogen bonds, reproduced from [2]:
asymmetric unit of the crystal structure of ammonium carbonate monohydrate

Wikipedia[3] therefore offers the best possible Lewis structure:
Lewis structure of ammonium bicarbonte

Strictly speaking a Lewis structure may only contain covalent bonds, i.e. two-centre-two-electron bonds, while ionic bonds are implied by placing the (molecular) ions in close proximity to each other. However, the hydrogen bond is a well known concept in chemistry and it was already extensively discussed by Linus Pauling. In his book "The Nature of the Chemical Bond" he uses dashed lines to represent the hydrogen bond.[4] Certainly another interesting read is the approach of a more unified theory about the hydrogen bond.[5]
Another possibility is to introduce the donor-acceptor notation with arrows, to indicate a bonding interaction. Therefore one of the following structures may also be considered correct in an extended Lewis concept.

alternative Lewis Structures (in an extended Lewis concept)

  1. The following article reads like this is still a work in progress: R. Brooks, and T. C. Alcock, Nature, 1950, 166, 435-436. (It does also not come with any supporting information.
  2. A. D. Fortes, I. G. Wood, D. Alfè, E. R. Hernández, M. J. Gutmann, and H. A. Sparkes, Acta Cryst. Sec. B, 2014, 70, 948-962. This quite interesting article is open access.
  3. Ammonium bicarbonate, Ammonium carbonate
  4. Linus Pauling: The Nature of the Chemical Bond. Cornell University Press, 1960.
  5. G. Gilli, P. Gilli, J. Mol. Struc., 2000, 552 (1-3), 1-15.
  • $\begingroup$ My chemistry is bad. Why is the bond in nitrogen dioxide allowed? $\endgroup$ May 26 '15 at 7:01
  • $\begingroup$ Well, that cannot be answered in one or two sentences, as it refers to the stability of certain compounds. You may try ask a new question, but it might be too broad to be acceptable for this site. maybe you should read a little on bond formation and molecules first and then ask a question with specific points of what you did not understand. $\endgroup$ May 26 '15 at 7:10
  • $\begingroup$ Would you allow a variant of structure B, in which the order is oxygen, hydrogen, nitrogen, hydrogens, a.k.a. $\ce{HO-CO-OHNH3}$? Just for reference and clarity. $\endgroup$
    – Jan
    May 26 '15 at 15:20
  • $\begingroup$ @Jan At least I would consider this as more correct as structure B, but still incomplete. But Lewis structures hardly ever are. If you are true to the concept, then you cannot allow hydrogen bonds, which makes it quite impossible to construct any valid Lewis structure. But one can extend the concept, maybe I should include this... $\endgroup$ May 26 '15 at 16:02

They are certainly not the same. A is in dissociated form. Say in aqueous solution. B refers to not dissociated form but I have never come across non dissociated structure. There may not be actual bond between N atom and O atom.

  • 4
    $\begingroup$ "May not" is certainly not the right choice of words. There is no bond between oxygen and nitrogen $\endgroup$ May 25 '15 at 9:17
  • 1
    $\begingroup$ I was not sure sir thus I said 'may not'. $\endgroup$ May 25 '15 at 9:18

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