# Differences between H-H-O and H-O-H

I'm not an expert.

In simple words how do they differ:

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

$\ce{H_{2}O}$ 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...

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.

• 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. – Richard Terrett Jan 26 '13 at 8:53
• @RichardTerrett, thanks, +1. I'm at the undergrad level, so my knowledge of MO theory is limited. – Greg E. Jan 27 '13 at 1:28

I was completely unfamiliar with the notion of HHO before this question, so +1 just for bringing it to our attention.

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.