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Take this is an example:

$$\ce{HCl(aq) -> H^+(aq) + Cl^-(aq)}$$

and

$$\ce{HCl(aq) + H2O -> H3O+ + Cl^-(aq)}$$

Clearly the proton latches on to the water molecule but why? Also, why does the hydronium ion has an positive charge? Did the proton settle in the nucleus of the some atom?

What exactly is happening here?

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  • $\begingroup$ About the charge: 0 + (+1) = +1, i.e. water is uncharged, the hydrogen ion is charged, so the hydronium ion as the product of the two will be charged as well. It does not matter where exactly the protons and electrons are, the net charge will always be +1. $\endgroup$ Dec 16 '20 at 4:40
  • $\begingroup$ You may found many interesting info and some references in Wikipedia - hydronium page. See also that a hydronium ion is the most simple particular case of more general oxonium ions $\endgroup$
    – Poutnik
    Dec 16 '20 at 13:20
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The evidence of a positively charged hydronium ion or a simple hydrogen ion comes indirectly from electrolysis experiments. This was demonstrated by Arrhenius more than hundred years ago. When he proposed these ideas during his PhD work, his mentors did not like these ideas which are bothering you now.

If we take ultrapure pure water and dip two electrodes connected to a negative and positive terminals of a battery, no current passes through it. It means there are not many ions in water.

The moment you add HCl or any strong acid, water becomes a very strong conductor of electricity. As you might know in solution, electricity is carried by charged particles. This proves that there must be ions in an acidic solution. How do you we know hydrogen ion carries a positive charge? The simple demonstration is that hydrogen ion is attracted to the negative electrode- because hydrogen gas always appeared at the negative electrode.

Clearly the proton latches on to the water molecule but why? Also, why does the hydronium ion has an +ve charge? Did the proton settle in the nucleus of the some atom?

Well, this picture is still not very understood as far as I remember. The symbolism $\ce{H+}$ or $\ce{H3O+}$ is a very simple picture. Probably it is a solvated proton $\ce{H3O+.(H2O)n}$ but people have demonstrated spectroscopically that it exists. I do not know the fundamental and mechanistic reason as to why the HCl molecule when dissolved in water, it would protonate water. Yes, people can put forward thermodynamic reasons but thermo never talks about the mechanistic why's and how's.

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    $\begingroup$ But what seems to be missing in your opinion? Besides that we can write down H3O+ (even H+, if we keep it mind that is H+aq, where aq is specially strong) in places of the various hydrated species (3 water, 9 or whatever) what is missing from a mechanicist viewpoint, at list at textbook level? Does not Arrhenius acid base theory suffices? Thermodynamics only guarantees a net energy gain. (I am referring especially to your passage "I do not know the fundamental reason....". $\endgroup$
    – Alchimista
    Dec 16 '20 at 8:42
  • $\begingroup$ I wanted to say Lewis instead of Arrhenius, being the first one a mere requisite to the formation of hydronium(s) ions. $\endgroup$
    – Alchimista
    Dec 16 '20 at 13:22
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Let me explain it in a less complicated and direct way.

H+ 'reacts' with H2O to form the hydronium because the oxygen in the water molecule has a lone pair of electrons that attracts the H+ ion.

Why does it have an overall positive charge? It is something you will be comfortable seeing in chemistry. Right now you seem to be just starting to question why things are the way they are in Chemistry. You will gradually get used to the way things are supposed to be. The reason for why hydronium has a positive charge is similar to why ammonium has a positive charge. When atoms with lone pair, like oxygen in hydronium or nitrogen in ammonium form a coordinate bond, the atom having donated its lone pair gets a positive charge on it instead of the H+

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    $\begingroup$ It can be also said a proton is an extremely strong Lewis acid and reacts with H2O as a Lewis base, attaching itself to the free electron pair of H2O. $\endgroup$
    – Poutnik
    Dec 16 '20 at 11:23
  • $\begingroup$ And thermodynamics turns useful, otherwise one can argue why HCl dissociate in water if charges are inherently less stable than HCl itself. But perhaps it is a trivial remark. $\endgroup$
    – Alchimista
    Dec 16 '20 at 13:27
  • $\begingroup$ "hydronium because the oxygen in the water molecule has a lone pair of electrons that attracts the H+ ion" This does not make sense at all. $\endgroup$
    – M. Farooq
    Dec 16 '20 at 15:11

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