Close Packing

Close packing produces two distinct layer arrangements. In hexagonal close packing, the third layer lies directly above the first, creating an ABAB sequence. In cubic close packing, the third layer occupies a new set of hollows, producing ABCABC stacking. Thus, the structures differ in long-range layer order even though their local packing efficiency and nearest-neighbor count are the same.

Each particle contacts six neighbors within its own layer and gains three contacts from the layer above plus three from the layer below. This gives a coordination number of 12, meaning twelve nearest neighbors surround each particle. The result describes the local coordination environment and helps chemists compare how particles are arranged within different crystal lattices.

The spaces between close-packed particles form interstitial sites with different geometries. Tetrahedral voids are surrounded by four particles, whereas octahedral voids are surrounded by six. These sites provide a structural framework for analyzing where smaller ions or other particles may be represented in ionic solids and for describing coordination environments within crystal models.

Packing efficiency depends on how much of the available volume is occupied, not solely on the order of successive layers. Both HCP and CCP place each new layer into hollows that minimize unused space, giving an efficiency of about 74%. Their different ABAB and ABCABC sequences therefore change the lattice arrangement without changing the idealized volume fraction or coordination number.

Begin with a layer of equal-sized particles arranged so each particle contacts neighboring particles. Place a second layer in the hollows of the first, then choose either the original hollow alignment for an ABAB sequence or a different hollow set for an ABCABC sequence. Repeating the selected pattern builds the three-dimensional lattice and reveals its coordination and voids.

In chemistry, close-packed models organize the structural description of ionic solids and metals by showing particle contacts, coordination environments, and available interstitial spaces. For ionic materials, the voids help represent possible positions within the lattice. For metals and related materials, the model provides a way to discuss how equal-sized particles occupy space in an ordered crystal.

A model can identify whether the arrangement follows HCP or CCP stacking, determine the twelve-neighbor coordination environment, and locate tetrahedral or octahedral voids. These structural features give researchers a common framework for comparing crystal arrangements in alloys and other materials. The model focuses on geometric organization and space filling rather than chemical identity alone.