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I'm not an expert.

In simple words how do they differ:

$\ce{H-H-O}$ and $\ce{H-O-H}$ molecules?

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$\ce{H_{2}O}$ is the water molecule, two hydrogen atoms attached to a central oxygen atom, $\mathrm{C}_{2v}$ symmetry, thermodynamically minimal structure of these atoms, Adam's ale, etc.

HHO is a poorly defined term often bandied around by 'water powered car'... enthusiasts. I'm not sure that it's supposed to represent a molecule so much as a state of hydrogen and oxygen that conventional chemistry has no concept of.

I think we're supposed to believe that it's a gas of hydrogen and hydroxyl radicals, or of atomic hydrogen and oxygen, or protons and hydroxide ions, or something, that is metastable at room temperature and that also happens to violate the first law of thermodynamics when produced and combusted. Why this glaringly obvious anomaly in quantum electrodynamics and thermodynamics has never been observed anywhere in nature before is anyone's guess (probably part of the conspiracy).

These guys should write up a few papers, get published in Nature, collect the Nobel chemistry, physics and peace prizes...

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There is no molecule in existence with the structure H-H-O, for the simple reason that hydrogen possesses only one orbital and is therefore chemically incapable of forming more than one bond or maintaining more than two electrons in its orbit. Therefore, the formula $\ce{HHO}$ is either a very idiosyncratic way of denoting a molecule of water (normally written $\ce{H2O}$ and occasionally $\ce{HOH}$, in order to emphasize its structure, i.e., H-O-H), or it refers to oxyhydrogen, which isn't actually a molecule at all, but rather a mixture of hydrogen and oxygen gases (the molecules $\ce{H2}$ and $\ce{O2}$, respectively) used as a fuel.

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    $\begingroup$ To add to this, there are exceptions to the 'hydrogen only makes one bond' rule of thumb, in the form of unusual electron-deficient bonds as found for instance in boranes, however this does not apply to 'H-H-O'. These bonds cannot really be reconciled with valence bond theory and require molecular orbital theory to make sense. $\endgroup$ – Richard Terrett Jan 26 '13 at 8:53
  • $\begingroup$ @RichardTerrett, thanks, +1. I'm at the undergrad level, so my knowledge of MO theory is limited. $\endgroup$ – Greg E. Jan 27 '13 at 1:28
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I was completely unfamiliar with the notion of HHO before this question, so +1 just for bringing it to our attention.

I don't have much to add to the previous answers, but for those interested in learning more about the origins of HHO (and if you have access to these journals through your university), head on over to:

  • The main article by Santilli in the International Journal of Hydrogen Energy volume 31 (2006) pages 113-1128.
  • A discussion by J. M. Cato in the same journal, volume 32 (2007) pages 1309-1312 which points out some of the issues with interpreting the data in the main article. (Note there's some great pedagogical opportunities in this work for General Chemistry courses: the Santilli paper uses non-SI units that can be converted, and the basic thermochemistry of evaporation and oxidation/reduction reactions can be incorporated into Hess' Law type activities.)
  • There are two follow up discussions, in the same journal again, one by Cloonan volume 21 page 1113 and one by Kadeisvili which serve as a rebuttal to Cato's arguments.

This whole discussion makes for a great activity on the scientific method, understanding of analytical techniques, proper interpretation of data and the inherent resistance to change within the scientific community. This reminds me of the cold fusion debacle, which is very well described in Gary Taubes book Bad Science. Good reading for anyone interested in these types of controversial experiments.

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The structure $\ce{H-H-O}$ does not technically exist in any common conditions because hydrogen does not generally form two covalent bonds at once. Such a structure would require a TON of energy to be put in because the hydrogen nucleus's lone proton would need to be able to hold electrons in the $2s$ sub level and prevent them from leaving the atom's circumference. Oxygen however readily forms two covalent bonds making $\ce{H-O-H}$ a very chemically plausible and common structure. Why can oxygen form two covalent bonds? Think of it in terms of quantum mechanics, much in the way as I explained for hydrogen.

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