Striking an NaCl crystal cleaves it cleanly along flat faces, while an equivalent blow merely dents a copper sheet. Why is the ionic solid brittle?

Core Concept

Ionic stability depends on strict alternation of positive and negative charges through the lattice. A hammer blow displaces one plane of ions by a single position, setting Na+ against Na+ and Cl- against Cl- across the contact. The abrupt like-charge repulsion drives those planes apart, so the crystal fractures along a flat surface instead of deforming. Copper avoids this failure because its electron sea keeps bonding intact however the atoms shift.