# How can ionic substances have an infinitely-repeating lattice structure?

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.

• What happens near the edges? – drunkBrain Mar 19 '15 at 12:18
• Edge effects and defects are indeed important in some cases, particularly on the nanoscale. – Geoff Hutchison Mar 19 '15 at 17:07

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.