Final answer:
Metals are malleable and ductile owing to free-flowing electrons that allow metal ions to slide past each other, while ionic-crystalline compounds are brittle because of rigid ionic bonds that shatter under force.
Step-by-step explanation:
Metals are malleable and ductile due to the presence of free-flowing electrons that act as a lubricant, allowing metal ions to slide past each other without breaking the metallic bonds. This can be likened to ball bearings coated with oil. In contrast, ionic-crystalline compounds like NaCl are brittle because when force is applied, like-charged ions are forced together, which disrupts the ionic bonds and causes the crystal lattice to shatter.
The different behavior in metals and ionic compounds arises because of the nature of their chemical bonds and structures. Metals have a 'sea of electrons' that allows for the movement of atoms upon applying force, preserving the structure even when reshaped. On the other hand, ionic bonds in ionic compounds are much more rigid and when subjected to stress, the lattice collapses, leading to the compound shattering instead of deforming.