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Why are the oxides usually insulating? It would be nice to get an explanation with chemistry terms (e.g., electronegativity, electron affinity, etc...) and a different explanation involving the language of band structure (i.e. energy gaps).

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    $\begingroup$ Without defining the temperature which you have in mind, the question is meaningless. Oxides are not metals, in the first place, and as non-metals most of them have a rather big gap in electron band. So, most of them are isolators at room temperature and semiconductors at appropriate themperatures or when doped with some foreign elements. $\endgroup$ – Georg Apr 16 '13 at 11:19
  • $\begingroup$ He didn't even say whether he meant thermally or electrically insulating- which one? $\endgroup$ – user2617804 Nov 10 '13 at 12:07
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    $\begingroup$ I meant electrically insulating and at room temperature. Perhaps my question would be best worded as, "Why do oxides tend to have large band gaps?" $\endgroup$ – ChickenGod Nov 12 '13 at 2:22
  • $\begingroup$ I don't see how would one able correctly describe band properties without band structure, based on some chemical concept rather limited. Maybe some simple, and somewhat wrong explanation like "they have ionic bonds due to large electronegativity difference between metal and oxygen". Note that there are small bandgap oxides, semi-metals, even metals. Some, like $ReO_3$ are highly conducting that cannot be explained by electronegativity. $\endgroup$ – Greg Sep 10 '14 at 2:44
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To have conductuing valence zone requires to have infinite amount of ovarlapping valence orbitals with lesser amount of electrons. This is not usually possible for oxides, because they contain direct contacts O-O, but not M-M. O-atoms in oxides has completely filled electronic shell. Pure metals has electronic shells with much less electrons then available orbitals so they can allow electrons to move freely. This is the difference.

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    $\begingroup$ I am aware our constrains here, but I think it would be better to use at least the concept of unit cell, so the explanation can go beyond "infinite number of orbitals" and such. I hope you noticed that even a single atom has infinite number of orbitals, and that the reason of insulation is NOT the lack of overlap in most insulating materials. $\endgroup$ – Greg Sep 10 '14 at 2:50

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