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Why is H3O positively charged?

  1. Oct 19, 2012 #1
    [H3O]+ has an extra electron, so shouldn't it be negatively charged?

    Similarly, [NCO]-, with (for simplicity sake) a ressonance structure of N=C=O means N needs an electron, and thus the whole molecule is short of one electron. Doesn't that mean it is positively charged?
     
  2. jcsd
  3. Oct 19, 2012 #2
    No it does not have an extra electron. Do the electron accounting and apply the octet rule.

    Do you think that 100 years of PhDs in chemistry and chemical engineering are dumb and missed that H3O was negative?
     
  4. Oct 19, 2012 #3
    Is this lewis dot not right?

    Oxygen has 9 electrons when it should only have 8 to satisfy octet rule...
    That means it has an extra electron than it needs to be stable, and by definition, is an anion. Because oxygen has an extra electron, so does the whole molecule. Here is a picture... is this right?
     

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  5. Oct 20, 2012 #4

    AGNuke

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    Oxygen has 9 electrons? Gimme a Break!!! First find out how many electrons Oxygen ACTUALLY have? How many of them are in valence shell.
     
  6. Oct 20, 2012 #5
    Oxygen has 6 atoms in its valence shell. It needs 2 more to be stable and satisfy the octet rule. If it was H2O it forms a covalent bond with hydrogen and both are stable. With H3O, there is an extra hydrogen so if it forms a covalent bond with that too, then the extra hydrogen is satisfied, but oxygen has one more electron than it had in H2O. So it has 9 total. Is this right?
     
  7. Oct 20, 2012 #6
  8. Oct 20, 2012 #7
    Yes, but in the diagram it shows Oxygen has only 5 electrons (including the lone pair) to itself before bonding. Doesn't oxygen have 6 in the valence shell?
     
  9. Oct 20, 2012 #8
    What do you mean "before bonding?" The tutorial presents the structure of the hydronium ion and how to calculate the formal charge on each atom. Are you referring to the formation of hydronium ions due to water dissociation? One has a neutral water molecule and a hydrogen cation (due to some other water molecule dissociating into H+ and OH-) forming the hydronium ion.
     
  10. Oct 21, 2012 #9

    Redbelly98

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    As you correctly point out, oxygen should have only 8 electrons (including those it shares with the hydrogen atoms) to satisfy the octet rule of stability. So ... there is no orbital for that extra electron to the lower left of the oxygen. That electron will not be part of this system, and you end up with a missing electron. What is the charge on a system that is missing one electron?

    By the way, why do you think including that extra electron in your figure would result in a -1 charge? Count up all the protons and electrons, and see what you really get. Remember to include the 2 inner-shell electrons for oxygen that are not explicitly drawn.

    He was counting the electrons being shared with the hydrogens. Counted that way, it should be 8 from the octet rule.
     
  11. Oct 21, 2012 #10
    Take an oxygen atom with 6 valence electrons. Remove one to make O+ with 5 valence electrons. Now make covalent bonds with three hydrogen atoms each donating one electron into the bonding, and you have H3O+, with 8 valence electrons around the oxygen, when you count the three that are shared from the hydrogen atoms.

    And there you are!
     
  12. Oct 21, 2012 #11
    Thanks! That makes sense. It is neutral when including the extra electron, but H3O cannot exist with that extra electron because oxygen has one over the octet, so it must give away that electron, resulting in a positive charge, and thus a positive charge on the whole molecule.
     
  13. Oct 22, 2012 #12
    Uh, I'm not sure that's exactly the best way to think about it. First off, the "octet rule" isn't really a rule, it's just a sort of mnemonic device to relate some observations for a section of the periodic table. Nothing happens "because of it". Also, I'm not sure if the picture you're having of a neutral hydronium complex forming and then an electron being lost is correct. I'm not sure it's wrong either, but my guess would be the scenario mentioned above of forming the proton and the OH- and then forming hydronium via H+ + H2O --> H3O+ .
     
  14. Oct 24, 2012 #13
    I hope I am not digressing but often when students ask questions like these, they are trying to find out the reason for something which they don't clearly understand; they are not trying to question the validity of well established facts. (speaking from personal experience)
     
  15. Oct 24, 2012 #14
    sorry a few trolls annoyed me off that day.
     
  16. Oct 24, 2012 #15

    Redbelly98

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    I think krackers now has a reasonable understanding, given that this looks like introductory level.
    Yes, but this is true of much that is taught at the introductory level.
    krackers did say "but H3O cannot exist with that extra electron" in post #11. I think it's okay to include that electron as an intermediate step toward finding the actual configuration. Of course, one could also simply NOT include that electron in the first place; just fill in shared pairs and lone pairs around the oxygen to get 8 electrons, and do a final count (including hydrogen) to get the net charge.
     
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