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We have $\ce{H2O}$ with two lone pairs on the oxygen atom and $\ce{H3O+}$ with only one lone pair on the oxygen atom.

One electron from the four non-bonding electrons has made a bond with the third hydrogen atom, so we should still have three electrons left. Why do we have only two?

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    $\begingroup$ What is the charge of $\ce{H3O+}$... $\endgroup$
    – bon
    Oct 10, 2017 at 11:00

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The answer is quite simple: When you have $\ce{H2O}$, it means you have two hydrogen and one oxygen atoms. Each hydrogen brings 1 electron, while the oxygen brings 6.

Your assumption now is, that you have another hydrogen atom bringing in another electron. In this case, you'd be right to assume that there are three electrons from hydrogen, totaling up to 9 electrons in the molecule. But this would violate the octet rule and consequently Paulis exclusion principle.

The solution to your question is, that you do not have $\ce{H3O}$, but $\ce{H3O+}$! The plus means, there is one electron less in the molecule, because you don't bring another H (with one electron) into the molecule, but rather just a proton ($\ce{H+}$ with the electron removed).

So, you have 6 electrons from the oxygen atom and only two from the hydrogen atoms, totaling up to 8 electrons. Three covalent bonds take up 2 electrons each, leaving you with two electrons in the remaining sp3 hybrid orbital as the lone pair.

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  • $\begingroup$ If we brought a proton, how did it connect to the O, it doesn't even have electrons to share?! and when drowing Lewis structure the O may have a positive formal charge, and the H may have no formal charge, why, if its a proton? $\endgroup$
    – Muhammad
    Oct 10, 2017 at 16:59
  • $\begingroup$ Well, it only formally is a proton. Actually, there are no protons in aqueous solutions. $\ce{H+}$ is usually just short for $\ce{H3O+}$. But that is not important to answer the question. It is important, however, to count the available electrons and deduct 1 for the positive charge! $\endgroup$
    – basseur
    Oct 10, 2017 at 17:23
  • $\begingroup$ Another thing: the electrons for a covalent bond do not have to come from both atoms, but can also be completely donated by only one (here the oxygen atom). The only important thing is that there are two electrons forming the bond In the end. $\endgroup$
    – basseur
    Oct 10, 2017 at 22:01

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