Having looked at the various definitions of acids and bases and having refined my understanding of it after learning about the inadequacies of pKa and the novel use of the Hammett acidity function, I would like to ask if the bare proton is the strongest acid?

I would like to define "acid strength" not as the extent of dissociation but more simply as the ability to protonate other chemical species.

I have come to this conclusion after reading through a post on the explanation behind the strength of fluorantimonic acid, being that the bare proton is liberated and that the conjugate base is so well coordinated, allowing the charge to be spread out over a large structure, stabilising it to a great extent.

There is no doubt about the proton being the strongest acid in the Brønsted-Lowry sense. Similarly, in the Lewis sense, this should also be logical as what could possibly more electrophilic than a bare proton?

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    $\begingroup$ An acid is a proton donor , not the proton itself $\endgroup$
    – Technetium
    Jul 2, 2017 at 16:46
  • $\begingroup$ Related answer by @Geoff about the strength of acids (or difficulty) here $\endgroup$
    – NotEvans.
    Jul 2, 2017 at 16:51
  • $\begingroup$ @NotEvans. It was only briefly mentioned in the comments and was not touched on in detail. Also, I am assessing the ability to protonate, not the other criteria (i.e. gas phase proton affinity, Hammett acidity function) they have mentioned. $\endgroup$ Jul 2, 2017 at 16:55
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    $\begingroup$ Problem is there's only kinda "bare" H+ even in fluoroantimonic, not really bare. You need sth like particle beam to have bare protons. $\endgroup$
    – Mithoron
    Jul 2, 2017 at 17:32
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    $\begingroup$ chemistry.stackexchange.com/questions/72075/… $\endgroup$
    – Mithoron
    Jul 2, 2017 at 18:05

3 Answers 3



Brønsted theory

In Brønsted theory $\ce{H+}$ isn't an acid at all. Acids lose protons, becoming conjugate bases, and $\ce{H+}$ is the proton itself.

Arrhenius theory

$\ce{H+}$ isn't an acid, because in this theory acids dissociate in water to form hydrogen ions.

Lewis theory

$\ce{H+}$ is an incredibly strong acid, but nuclei of other, heavier elements, for example alpha particles, are arguably stronger. I haven't found hard data for this and it may be rather difficult to get, these aren't your friendly neighbourhood Lewis acids ;)

Protonating agent

Bare $\ce{H+}$ might be the ultimate protonating agent. In proton transfer, with any Brønsted acid you could always try to find an acceptor weak enough that reaction constant would be lower then 1, proton would "prefer" to stay with acid then protonate base. That's not the case with bare proton, which is unbound. Therefore it may beat any Brønsted acid.

Why only "might"? Because whether bare proton can bind to a species depends on it's energy, which needs to be lower then proton affinity of a molecule to which it's supposed to bind. Otherwise proton may ionise the molecule instead, and even fuse with one of its nuclei, if energy is high enough.

Another thing is that acids like $\ce{H4O^2+}$, which are endothermic molecules, could beat a bare proton, because of their repulsive nature - they spontaneously lose protons and "throw them away" with positive charge of their conjugate base!

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    $\begingroup$ Ok got your point. What about protonating ability? Would it be the strongest in terms of the ability to protonate other species? Perhaps I wasn't clear enough, but I am not looking for an analysis with respect to the different definitions. But rather in terms of protonating ability only $\endgroup$ Jul 3, 2017 at 9:42
  • $\begingroup$ It may be questionable, as an object can provide itself. OH- would not be an Arrhenius base either. $\endgroup$
    – Poutnik
    Nov 22, 2020 at 11:55
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    $\begingroup$ @Poutnik Indeed it is not. Arrhenius theory is about chemical compounds and H+ or OH- aren't compounds, but species being dissociated from compound. Having a proton beam pretty much breaks down a theory, but it's not much of a problem and it hardly protonates anything, instead ionising, breaking bonds etc. $\endgroup$
    – Mithoron
    Nov 27, 2020 at 15:14

Interesting question. I'd say that the strongest acid would have to be something that cannot be protonated. Probably something like HeH(+). Helium is so noble it shouldn't be protonated at all although you can make this compound in the gas phase it still is considered to be one of the strongest acids.

  • $\begingroup$ Could you please substantiate your answer? I have never heard of this perspective before. $\endgroup$ Jul 31, 2017 at 2:30
  • $\begingroup$ Take water for example, the conjugated bases of strong acids (superacids) cannot be protnated by the solvoacid (H3O+). If you switch to different systems it is possible using Carborane acids for example to even protonate Benzene. So for the OP's question I assume a solvent or gasphase, which extends really far towards the superacidic site and then. Here a super strong solvoacid can be predicted, which protonates anything. But what is the limit for protonation? The proton affinity. The lower the harder it is to protonate it, the more easily it will give the proton of, which is called an acid. $\endgroup$ Jul 31, 2017 at 5:43
  • $\begingroup$ Certainly Helium, being the most unreactive (although I am not sure whether Neon might be even more unreactive) element or compound is a super stable conjugated base of HeH(+). So if you are able to make HeH(+) this should easily try to give off it's Proton again. $\endgroup$ Jul 31, 2017 at 5:48

There is a very very old "paradoxical" question: "Which came first: the chicken or the egg?" If all chicken eggs come from chickens and all chickens come from eggs, then there is a paradox of causality.

Your question, after including the various comments you've added to it, has a similar problem: Is H+ an H+ donor? You appear to believe it is, I disagree. It is a matter of definition, so we can each hold mutually exclusive definition and self-consistent belief about what does and does not constitute an "acid". I don't know one way or the other whether your definition is self-consistent. I would argue it isn't very useful and also point out to you that you are not (nor am I) an authority in this field. Given that, I suggest you (re)consider whether you are competent (knowledgeable and experienced) enough to be creating your own definitions for technical terms.

The "bare" proton exists in some environments. (Of course, it exists even in the vacuum of space at a minimum surrounded by all sorts of quantum fields so isn't "really" bare.) These situations are not often (never as far as I know) used in the Chemical literature in discussions of acid/base strength for the obvious reason that they involve (arguably) more Physics than Chemistry. The bare proton doesn't exist in solution (by definition) and it's well known that a proton solvated in liquid ammonia will have different "strength" than one solvated in water.

Which gets us to the question that needs to be answered clearly before you go promoting your definition of acids to include H+: What do you mean by the term "H+"?

  • $\begingroup$ When I mean "proton", I mean an unsolvated proton. Assume that it can exist theoretically exist. $\endgroup$ Jul 31, 2017 at 2:32
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    $\begingroup$ So can an alpha particle, which has two positive charges and no electrons. And it isn't just theoretical. You probably have a few of them floating around in your smoke detector. $\endgroup$ Mar 3, 2018 at 12:00

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