atomic structure

Atomic structure, in this field, means the arrangement of atoms at the very finest scale, which atoms sit where, and how they are packed relative to their immediate neighbors. Picture atoms as tiny spheres: are they stacked in a neat, endlessly repeating pattern like oranges in a grocer's crate, or piled at random like sand in a bag? That answer, the atomic-scale arrangement, is the base layer of the whole structural hierarchy, the level from which every larger feature is built.

Two things fix the atomic structure: what the neighbors are (the local bonding and packing, described by the coordination number, how many nearest neighbors an atom has) and whether that local packing repeats regularly over long distances. If it repeats periodically we call the material crystalline, and the repeat is captured by a tiny building block, the unit cell (typically 0.3 to 1 nm on a side). If it does not repeat, the material is amorphous. A worked example: in face-centered-cubic copper, each atom touches 12 neighbors (coordination number 12) and the packed spheres fill 74 percent of space.

Atomic structure sets the properties that come straight from bonding and packing, stiffness, density, and often optical and electronic behavior, and it is the stage on which defects and larger features later appear. It is measured chiefly by diffraction. A clarification worth making early: do not confuse atomic structure in materials (where the atoms sit in the solid) with the internal structure of a single atom (its nucleus and electron shells), the same words, a different question. This field means the former.