Way in which atoms (considered as hard spheres) pack together to fill space. In cubic closest-packing, there are three alternating hexagonal layers, a, b, and c, offset from one another so that the spheres in one layer sit in the small triangular depressions of neighboring layers. This Definable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with of packing is subtly different than cubic closest packing. Each sphere is touched by 12 neighbors, 6 in the same layer, 3 in the layer above, and 3 in the layer below.
Some or all content above used with permission from J. H. Wittke.