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I know this might be a bit of a silly question, but what is a hydrogen bond? I always thought that (say in water for example) the hydrogen was slightly more negative than the other ion (oxygen). But hear me out, this is because the shared electron (between the hydrogen and the oxygen due to their covalent bond) is slightly closer to the hydrogen's positive nucleus (than the oxygen's positive nucleus).

And since both are gaining an electron, that additional electron had more of an effect on the overall hydrogen ion (and less so of an effect on the oxygen ion) than a charge of exactly -1. Therefore if the charge of the hydrogen ion is now is slightly less than -1 and the charge of the oxygen is slightly more than -1, then the hydrogen is more negative than the oxygen...

Wait, if both the hydrogen and the oxygen ions are negative, why would the attract other negative ions from other water molecules, surely they'd repel. Obviously there is some flaw in my understanding of this and im just hoping someone can help me out.

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    $\begingroup$ en.wikipedia.org/wiki/Hydrogen_bond $\endgroup$
    – Karl
    Jan 2, 2017 at 20:12
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    $\begingroup$ H in H2O has a partial "positive" charge and not "negative" charge. The Oxygen atom has a partial negative charge. $\endgroup$
    – MrAP
    Jan 2, 2017 at 20:13
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    $\begingroup$ chemistry.stackexchange.com/questions/49961/… $\endgroup$
    – Mithoron
    Jan 2, 2017 at 20:21
  • $\begingroup$ @MrAP What does the electronegativity mean and why does mean that the oxygen has to 'hog' the shared electrons (giving it its partial negative charge)? $\endgroup$
    – Alex P
    Jan 2, 2017 at 20:54
  • $\begingroup$ Electronegativity refers the tendency of an atom to attract electrons towards itself. If the atoms forming a molecule of a compound have a high electronegativity difference then an ionic bond will be formed between them; if the difference is moderate then the bond will be a polar covalent bond, with the more electronegative atom attracting the shared electron pair towards itself and hence has a partial negative charge while the other atom has a partial positive charge, i.e., there is a charge separation. If the difference is low then the bond formed will be a non-polar covalent bond. $\endgroup$
    – MrAP
    Jan 2, 2017 at 21:19

2 Answers 2

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I always thought that [in water] the hydrogen was slightly more negative than the [oxygen]. […] [T]his is because the shared electron (between the hydrogen and the oxygen due to their covalent bond) is slightly closer to the hydrogen’s positive nucleus (than the oxygen’s positive nucleus).

You do not consider the relative size of atoms to determine their partial charges. It is not relevant by itself whether an atom is large or small; the only thing that matters is how well the nucleus can attract electrons — this is a value tabulated as electronegativity. The size of an atom in itself, and therefore also the distance from middle of a covalent bond to the nucleus of either atom is irrelevant; however, the size of an atom plays an indirect role since additional shells of core electrons reduce electronegativity by shielding the positive charge of a nucleus. It is not required to perform a formal analysis of how much influence the core electrons have, since — as I mentioned — the elements’ electronegativities are tabulated in periodic tables and similar resources.

Going strictly by electronegativity, oxygen has $3.5$ and hydrogen has $2.1$. Thus, the bonding electrons are polarised much more towards oxygen; this means that oxygen has an electron surplus (partial negative charge) while hydrogen has an electron deficit (and therefore a partial positive charge).


Another thing to consider is that a water molecule is a neutral molecule. Therefore, positive and negative partial charges must be somewhat evenly distributed. There is no overall surplus of electron, i.e. there is no overall charge. For an overall charge, you would have to turn to species such as the hydroxide ion $\ce{OH-}$. However, even therein the hydrogen is partially positively charged due to oxygen’s much larger electronegativity. This is not always given: in a peroxide ion $\ce{O2^2-}$, each oxygen atom will carry one single negative charge to give a symmetric molecule.


Taking this all into account, it makes sense to consider a hydrogen bond as an electrostatic interaction between a positively charged hydrogen bound to a negatively charged atom and another strongly electronegative, negatively charged atom of a different molecule.

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  • $\begingroup$ I understand most of that, but with the peroxide ion, I would've thought that each oxygen atoms have an additional electron each, making two oxygens with a charge of negative one each, making the peroxide ion have a charge of -2. But if you look at the two oxygens collectively, there are 14 electrons and 16 protons, so the overall charge must be +2. $\endgroup$
    – Alex P
    Jan 2, 2017 at 22:13
  • $\begingroup$ I understand that in a molecule the oxygen should be considered an atom rather than an ion, but if it's has more electron (8) than it would natural have (7) then why is it not a negatively charge ion, as opposed to a neutral atom? $\endgroup$
    – Alex P
    Jan 2, 2017 at 22:16
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    $\begingroup$ @LucasDyson-Diaz an entity is only called an ion if it features a full charge. Also, an atom can only be an ion by itself if it is not part of a molecule. A molecule can only be a molecular ion in its entirety; individual atoms of a molecule are never considered ionic. $\endgroup$
    – Jan
    Jan 2, 2017 at 22:29
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    $\begingroup$ @LucasDyson-Diaz Concerning the charges of the atoms in peroxide (and that of the entire ion): you're miscounting. Two oxygens at eight electrons each plus two additional electrons totals at $18 = 16 + 2$. This can also be derived from the ion's Lewis structure. $\endgroup$
    – Jan
    Jan 2, 2017 at 22:34
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The assumption you are making that is leading you astray is that hydrogen, covalently bonded to a more electronegative atom (hydrogen bonding is not exclusive to oxygen), is negatively charged. A partial positive charge that interacts intermolecularly with neighboring molecules resides on the hydrogen atom, at which electrons reside less frequently than the Bohr model would suggest.

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  • $\begingroup$ Sorry @David Shaked, I'm saying that the hydrogen is negatively charge (which you say is wrong) and you're saying that it is in fact "partially positively charge". How do you get to understanding that? I would've thought that if an electron is being gained to a neutral hydrogen atom (since electrons are negatively charged) then the hydrogen ion become negative. $\endgroup$
    – Alex P
    Jan 2, 2017 at 20:28

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