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So I'm studying for a chemistry test and one of the practice problems is "What is the intermolecular force present in water?" We made handy flow charts for this and so I determined that it was polar. The next step asks me to use a Lewis dot diagram to find the shape; symmetrical means London Forces and Asymmetrical means either Dipole-Diploe or H bonds. Easy enough; it's bent, which would make it symmetrical, right? The answer key says it has H-bonds and that makes sense, but I concluded that it had London Forces because bent is symmetrical? What am I missing?

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  • $\begingroup$ Even if it's the kind of bent where there are two extra electron clouds (which it is) that doesn't change the fact that it is symmetrical... $\endgroup$ – ConfusedScientist May 21 '16 at 19:35
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    $\begingroup$ Whether it has symmetry or not is completely irrelevant. Everything has London dispersion forces but in some molecules they are masked by stronger interactions such as hydrogen bonds. $\endgroup$ – bon May 21 '16 at 19:53
  • $\begingroup$ Strictly speaking, all: Lewis, dipoles, and H-bonding are present in water. I suspect that there's a item writing problem here (read=poor wording). If the question is about just IMF's, then all of them, if it asks for the $strongest$, then H-bonding. $\endgroup$ – ChemistryHelpCenter Aug 17 '16 at 17:32
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Any answer to your question depends on speculations about what your "flow chart" specifically is. You seem to confuse molecular shape symmetry with H-bonding. There is no simple relation between them. HOH using Lewis octet rules, gives the O atom two lone pair and so a bent bond angle is predicted. This in turn would suggest that water has a large dipole moment and so is a polar molecule. This says nothing about whether the H atoms or the central O atom can hydrogen bond. Water has two planes of symmetry: all three atoms (by definition 3 points define a plane) are in a plane so are symmetrical about that plane, and the two H atoms are mirror images in a plane through the central O atom and perpendicular to the first plane I mentioned. But so what? None of this speaks to whether either the H or the O atom form H bonds. The ability of an H to form an H bond depends on the Z-H bond polarity, NOT the polarity of the entire molecule. (where Z is the atom the H is (covalently) bonded to). Some Z-H bonds, for instance in methane, the C-H bonds make the H atoms very unlikely to H bond. Consider the chlorite anion ClO2(-) we can correctly predict structure using Lewis dot rules, we can predict that it will be polar (not to mention being negatively charged), but what can we predict about each atom's H bonding propensity? Nothing as far as I can see. Will any of the 3 atoms H bond? Well, since it is negatively charged, I'd expect it will. So, which of the atoms will do so? Lewis structure and symmetry don't help, as far as I can see. What would your flow chart predict (and what is the correct answer) ? IDK.

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