I was doing an acid-catalyzed mechanism and I ran into hydronium ($\ce{H3O}$) and I wasn't able to figure out why its structure is the way it is. If you look at the structure, you see the hydrogens, and an oxygen; however, the oxygen has two electrons remaining outside of it. This is confusing to me as If you add up all the electrons (3 from Hydrogens, 2 from the remaining oxygen), we have a total of 5 valence electrons, even though we should have a total of six. I would assume that each hydrogen would take up one electron from the oxygen each, thus three electrons would remain on the oxygen.

Could someone please explain why this is? Thanks.

Here is a little sketch I made to show what I mean:


  • $\begingroup$ Why should there be 6 valence electrons? There aren't 6 electrons around oxygen in, say, water. Have you heard of something called the octet rule? And do you know how to draw a dot cross diagram (Lewis structure) of $\ce{H3O+}$? A lousy mspaint mockup would suffice. $\endgroup$ – orthocresol Aug 25 '16 at 4:28
  • $\begingroup$ +orthocresol I have done a bit of a review of the Lewis dot diagrams, from a long time ago; thus, I would assume I may have some mess ups! I made a little diagram so that you can see what I mean $\endgroup$ – Dan Aug 25 '16 at 5:59
  • $\begingroup$ Okay. First things first: in $\ce{H3O+}$, the oxygen has a positive charge, so it should really have one electron less then what you drew. Next, if you count ALL the electrons in your diagram (minus the one that shouldn't be there), there are 8 in total: 2 from the oxygen lone pair and 6 from the O-H bonds (3 from oxygen, 3 from hydrogen). All in accordance with the octet rule. $\endgroup$ – orthocresol Aug 25 '16 at 6:32
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    $\begingroup$ There is no $\ce{H30}$, like you originally wrote. I assume you wanted to write $\ce{H3O}$, which still does not exist but at least gives us a hint as to where your problem is. The correct notation and formula would be $\ce{H3O+}$. Now what could the $+$ sign mean? ;) $\endgroup$ – Jan Aug 25 '16 at 9:26
  • $\begingroup$ +Jan Ah, I understand now! I was given the acid-catalyzed question verbally , and they didn't tell me that charge. Thank you so much! $\endgroup$ – Dan Aug 25 '16 at 15:24

Thanks to help from orthocresol & Jan, I was able to figure out my mistake. In the question, I was not given the charge nor the proper notation, and I was very confused. So anyways, the correct notation would have been $\ce{H3O+}$. Thus, there would actually only be two electrons remaining on the oxygen, and not three as I had thought.

Also, they had given me the octet rule (all electrons must add up to 8) which will definitely help me not run into this mistake again!



2 Hydrogen atoms would share their valence electrons with oxygen and forms covalent bond. The third Hydogen atom forms a coordination bond that is, it is actually a proton and both the shared electrons are given by oxygen. The net charge is +1.


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