[Decessus00]

I know that NaCl is a crystal lattice made up of Na⁺ and Cl^- ions.  Each Na⁺ ion is surrounded by six Cl^- ions, and each Cl^- ion is surrounded by six Na⁺ ions.  When you place two salt crystals next to each other, why don't the positive and negative ions of the one salt crystal attract the positive and negative ions of the other salt crystal and thus stick together?

[Arkcon]

Briefly, the crystal has ended, at the edge of the crystal.  Why has it ended?  What are the actual atoms at the edges?  That I don't know.  The simple explanation given is that there aren't always 6 atoms, surrounded by 6 other atoms, but instead flaws, caused by inclusions, that eventually add up to a flawless surface, that terminates.  It's hard to understand, but every crystal, must do this, or none of them would ever end.

[sjb]

I've often (well, not *that* often) wondered the same, and gradually talked myself into considering that the electrostatic forces tail off quite quickly, and under normal pressures the distance between the two faces is such that the attraction is not that much. Under higher pressures you may be able to get the faces closer together and so larger crystals may form.

Just my take.

S

[Borek]

the electrostatic forces tail off quite quickly

Exactly. From the outside crystal is perfectly neutral. To be able to 'feel' electrostatic attraction from either positive or negative ions you have to be very close.

IIRC electric field of a point charge goes down as r^-2, electric field of the dipole as r^-3, for more complicated objects (quadrupole and so on) it is diminishing even faster. Crystal is far more complicated than dipole.