I was learning about autoionization in science and my science teacher wrote $\ce{H3O+}$ as $\ce{H+}$ — now, my intuition would lead me to assume that:

$$\ce{H3O+} = \ce{H+}$$

But, $\ce{H+}$ is just an atom with a positive charge because its lack of electrons, while $\ce{H3O+}$ is the product of $\ce{H2O}$ nabbing that $\ce{H+}$, right?

So, is saying that $\ce{H3O+} = \ce{H+}$ just for the purpose of simplicity in the sense that we are assuming that in the solution (aqueous, is the context I am dealing with), the positive hydrogen atom (is it more specifically called an ion?) will bond with a water molecule?

P.S. I know that you would call $\ce{OH-}$ hydroxide, and $\ce{H3O+}$ hydronium, so what would you call $\ce{H+}$ (if it even has a name)?

  • $\begingroup$ @DavePhD, no - I am not asking whether or not the hydrogen ion exists, although that is a very intriguing question... that I will check out. $\endgroup$ Jun 7 '16 at 16:28
  • $\begingroup$ well after you read my answer there, explain what more you want to know $\endgroup$
    – DavePhD
    Jun 7 '16 at 16:30
  • $\begingroup$ When Hydrogen ion is solvated in water $\ce{H+_{\mathrm{aq}}}$, it gives$ \ce{H3O^{+}}$ $\endgroup$ Jun 7 '16 at 16:35
  • $\begingroup$ @DavePhD - Ok, I will see if any further clarification is needed. $\endgroup$ Jun 7 '16 at 16:39
  • $\begingroup$ @Jan - No, my question is regarding more formality rather than the actual chemistry behind it. $\endgroup$ Jun 7 '16 at 16:42

When dealing with an aqueous solution, you are correct that the $\ce{H+}$ ion is equivalent to $\ce{H3O+}$ for all intents and purposes. Due to the abundance of water in solution, molecules of $\ce{H2O}$ will readily pick up the hydrogen ions, meaning that most of the $\ce{H+}$ in an aqueous solution is actually of the form $\ce{H3O+}$. While this is the case, using $\ce{H+}$ over $\ce{H3O+}$ is more correct in general contexts. If you are using something like ethanol ($\ce{CH3CH2OH}$) as your solvent, then instead of having hydronium ions, you would have $\ce{CH3CH2OH2+}$ as your measure of concentration of acid.

As for your other questions, the $\ce{H+}$ would be considered an ion rather than an atom. This is because it is a charged species. Either $\ce{H+}$ can be referred to as a proton or as a hydrogen ion, since $\ce{H-}$ is too unstable a species to be formed. While some of your $\ce{H+}$ in solution might be deuterium or tritium, most people will understand what you are talking about if you just call the hydrogen ion a proton.

  • $\begingroup$ Great info! Thanks a lot - Just one quick clarification: Are you saying that we literally just call a hydrogen ion a proton in the specific context? e.g. Context: autoionization of water "Yo, dude, guess what? If a water molecule takes the proton from another molecule it makes hydronium!" $\endgroup$ Jun 7 '16 at 16:38
  • $\begingroup$ Yeah, that works. You can either refer to it as a proton or a hydrogen ion. $\endgroup$ Jun 7 '16 at 16:40

In water, $\ce{H+}$ never exists, the proton is always part of some other chemical (or ion). So, it would be a bit incorrect to say that $\ce{H+}$ bonds with water, at least in the sense that a reaction has occurred between a species best described as $\ce{H+}$ and something else (water).

I am tempted to claim that the answer by Bz is wrong, but it's reasonably close to being correct so will refrain. $\ce{H+}$ in water exists as both the $\ce{H3O+}$ ion and other $\ce{H + $n$\ H2O}$ species such as $n=2$ $\ce{H5O2+}$ and $n=3$ $\ce{H7O3+}$. These latter can be thought of as polymeric ions. (Because of charge repulsion, doubly negative species are a negligible component of liquid water.) The solvated (or aqueous) proton is generally just called the hydrogen ion, but other names are certainly possible. $\ce{H+}$ is the most common species in the Universe, but it doesn't exist in water. Since, for reactions in solution, the solvent is quite important, but beginning students are usually 'protected' from this level of detail, $\ce{H3O+}$ is a good way to express the concept how the $\ce{H+}$ (reactive) species is being 'carried' (or where it has gone to). Keep in mind that the hydrogen bonding that goes on in water is important for its physical and chemical properties. Any real reaction involving the transfer of $\ce{H+}$ in water will be similar to $\ce{XH + Y -> HY + X}$ (where these species may be charged or neutral (of course charge will balance) and definitely not involve $\ce{XH + Y -> XY- + H+}$, but the last could be $\ce{-> XY- + H3O+}$ (with the understanding that $\ce{H3O+}$ is shorthand for $n=1$ mostly with a bit of $n=2$ and less $n=3$, and less and less $n=4,5,6,7$).


Not the answer you're looking for? Browse other questions tagged or ask your own question.