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A distinct property of ionic compounds is that they are brittle. We are taught this is due to the fact that when force is applied to the lattice, ions are shifted, resulting in positive ions repelling positive ions and negative ions repelling negative ions. As a result, the lattice shatters. Hence ionic compounds are brittle. Makes logical sense to me.

However, using similar intuition, could we not just as well take the shattered pieces and hold them together again. At some point, we should get oppositely charged ions aligning causing the shattered pieces to come together again. Obviously when we hold, table salt together, they don't come back together. So what is fundamentally wrong with my intuition here?

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    $\begingroup$ It is not a distinct property of ionic compounds. Many molecular solids and even metals are also brittle. The causes of brittleness are far more complex than you think. $\endgroup$
    – matt_black
    Sep 29 at 11:45
  • $\begingroup$ chemistry.stackexchange.com/questions/78947/broken-object-bonds $\endgroup$
    – Mithoron
    Sep 29 at 14:46
  • $\begingroup$ chemistry.stackexchange.com/questions/109246/… $\endgroup$
    – Mithoron
    Sep 29 at 14:48
  • $\begingroup$ Also, the same-ion repulsion explanation of why ionic compounds are brittle is nonsense. Iodine crystals are also brittle, but the explanation clearly doesn't worth there. $\endgroup$
    – matt_black
    Sep 29 at 23:22
  • $\begingroup$ @matt_black Temperature equally plays a role, too. As in the lectures where a piece of flexible rubber, or an apple becomes brittle after their immersion in liquid nitrogen. $\endgroup$
    – Buttonwood
    Sep 29 at 23:37

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Your intuition is not wrong... but the atoms near the surface have to be free to move or exactly aligned, and must not covered by an adhered layer of air.

For example, by dissolving a salt in water and letting the water evaporate, you can watch crystals grow, as atoms (well, ions) find their place in the lattice. Read Crystals and Crystal Growing by Holden and Morrison, for example, download a free classic, such as Tutton's Crystals, or look online, such as Science Notes, How to Grow a Crystal.

Of course, there are crystals of metals and non-ionic compounds, too. Consider that only mildly-ionic water vapor condenses to make snowflake crystals, investigated by Bentley.

Simply holding pieces in your hand and hoping the crystal structure to align ignores theses issues:

  • The surface of the crystals would have to be atomically smooth, not jagged. Examine the surface of even a carefully cleaved crystal under an atomic force microscope and you'll find few smooth areas. The jagged peaks and valleys prevent the surfaces from meeting except at a few points.

  • As soon as a crystal is cleaved in air, the surface is covered with an adhered film. Germer states in The Study of Crystal Surfaces, "One of the most significant facts about real surfaces is that they quickly become dirty; a freshly cleaved crystal soon adsorbs foreign atoms on its surface." The effect is like using waxed paper to keep dough from sticking to surfaces.

    However, given smooth surfaces, they do stick together in the process of cold welding.

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