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Even when ionic compounds are strong (e.g. rubies and sapphires), they are neither malleable nor ductile in the slightest, and if sufficient stress is put on them, they will shatter, not bend. Why is that?

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    $\begingroup$ I'm not sure that I would characterize sapphire as an ionic compound. But, the question is a good one that points towards material science and metallurgy so I'll contemplate it. $\endgroup$ – Jon Custer Jun 25 '15 at 13:02
  • $\begingroup$ Fair about sapphires/rubies. But they were the hardest things with a metal and nonmetal I could come up with on short notice. $\endgroup$ – Breaking Bioinformatics Jun 25 '15 at 13:02
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Ionic crystals are hard because of tight packing lattices, say, the positive and negative ions are strongly attached among themselves.
So, if mechanical pressure is applied to an ionic crystal then ions of similar charges may be forced to get closer to each other.
Now, by doing so, the electrostatic repulsion can be enough to split or disorient completely the lattice infrastructure. Thus imparting the brittle character.

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Ionic compounds are brittle due to the presence of non directional bond between the ions.

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    $\begingroup$ Welcome to chemistry.se! Have a look around the site by taking the tour. This answer is very short, can you backup your claim with some references and further explanations. In its current state it is much more a comment than an actual answer. $\endgroup$ – Martin - マーチン Jun 25 '15 at 13:50

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