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The title's pretty much it. At school, we're taught that ionic substances are composed of an infinitely-repeating lattice, with atoms at fixed angles from other atoms. How is this possible when I can get an individual grain of salt? Clearly if there's an individual grain, it can't be infinitely repeating.

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    $\begingroup$ What happens near the edges? $\endgroup$ – drunkBrain Mar 19 '15 at 12:18
  • $\begingroup$ Edge effects and defects are indeed important in some cases, particularly on the nanoscale. $\endgroup$ – Geoff Hutchison Mar 19 '15 at 17:07
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To be technically accurate, it's "practically" infinite.

It's not infinite since as you mention a grain of salt (or even a large crystal) has some finite size.

You have to remember the actual atomic dimensions $10^{-10} \:\mathrm{m}$. So while it's not truly infinite, there are an incredibly large number of atoms in a crystal.

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    $\begingroup$ So it's essentially infinite in the middle, but what happens around the edges? Are there sodium ions with only three chloride ions attached to them, rather than the four it'd have with it in the middle? $\endgroup$ – drunkBrain Mar 19 '15 at 12:38
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    $\begingroup$ Yes, the only limitation is that the whole crystal is not charged. So as you said, there will be "there sodium ions with only three chloride ions attached to them". Not only that, there will be also here chloride ions with only three sodium ions attached to them. $\endgroup$ – ssavec Mar 19 '15 at 13:40
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    $\begingroup$ Side comment, physlink.com/Education/AskExperts/ae342.cfm estimated number of atoms in grain of salt to be 10E18. Therefore each edge is about cubic root of it long, about one million. Big, but not incredibly. But the surface effects do not reach deeper than ~5 layers, so the bulk of crystal is real crystal. (See history of en.wikipedia.org/wiki/X-ray_crystallography ) $\endgroup$ – ssavec Mar 19 '15 at 13:43
  • $\begingroup$ The difference between surface and interior properties is particularly notable in <a href="en.wikipedia.org/wiki/Topological_insulator">topological insulators</a>. $\endgroup$ – DrMoishe Pippik Mar 19 '15 at 14:32

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