The macroscopic behavior of a solid depends partly on how its atoms are arranged.

Crystalline solids

In a crystalline material, atomic positions have long-range periodic order. A lattice describes that translational periodicity, while a repeating unit cell is a convenient smallest building block from which the crystal can be generated by translation.

Common metallic crystal structures include:

• body-centred cubic (BCC);
• face-centred cubic (FCC);
• hexagonal close-packed (HCP).

Different structures have different neighbor geometries, packing, slip systems and directional properties even when they are built from atoms with similar bonding.

Amorphous solids

An amorphous solid lacks long-range translational order. Local neighbors can still have well-defined arrangements, but those motifs do not repeat periodically throughout the material. Glasses and many polymers are fully or partly amorphous.

Structure across scales

Crystal structure is only the first structural scale. Point defects, dislocations, grains, phases, pores and interfaces create additional structure that can dominate engineering behavior.

Chemical bonding helps determine which atomic arrangements are stable, but bonding mechanisms are general chemical knowledge and are treated separately from the geometry of crystal order.