Close packing is the most efficient way atoms or ions arrange in many solids, giving each particle 12 neighbors. In Inorganic Chemistry II, it shows up in metal and ionic crystal structures like FCC and HCP.
Close packing is the arrangement of equal spheres, such as atoms or ions, in the densest repeating pattern used to describe many solids in Inorganic Chemistry II. The basic idea is simple: each particle sits in a position that uses space efficiently, so the structure has very little empty room compared with looser packings.
The two standard close-packed layers are called A and B. In one layer, spheres sit in a hexagonal pattern. The next layer does not sit directly on top of the first. Instead, it drops into the gaps, or interstices, between spheres in the layer below. That offset is what lets the structure stay dense while building upward in three dimensions.
Stack those layers in one sequence and you get hexagonal close packing, or HCP, with an ABAB pattern. Stack them in a different sequence and you get face-centered cubic, or FCC, sometimes written as cubic close packing, with an ABCABC pattern. The names sound different, but both structures have the same packing efficiency of about 74 percent and the same coordination number of 12.
That coordination number matters because it tells you how many nearest neighbors surround each particle. In close-packed solids, each sphere touches 12 others, which is why these structures are so efficient. In real chemistry, that helps explain why many metals form FCC or HCP lattices and why certain ions fit into specific crystal arrangements.
Close packing is not just about drawing spheres on a page. It is a shortcut for predicting the geometry of a crystal, the shape of its unit cell, and how ions or atoms can fit together without wasting space. When you look at a metal like gold or a solid such as magnesium oxide, the close-packed idea helps you connect the 3D arrangement to density, stability, and some mechanical properties.
A common mistake is to think close packing means every solid is maximally packed. It does not. It is a model for the most efficient sphere arrangement, and many solids, including molecular crystals like naphthalene or network solids like quartz, do not fit this pattern in a simple way because bonding and shape matter too.
Close packing gives you a fast way to connect structure to properties in the solids unit of Inorganic Chemistry II. If you can identify an FCC or HCP arrangement, you can usually predict a coordination number of 12, a high packing efficiency, and a dense crystal lattice.
That matters when you compare metals, ionic solids, and more open frameworks. Gold is a classic example of a metal with close-packed structure, while magnesium oxide is often used to show how ions can fill lattice sites in a highly organized way. On the other hand, quartz and naphthalene remind you that not every solid is close-packed, because directional bonding or molecular shape can force a less efficient structure.
Close packing also gives you the language for describing unit cells and layer stacking. Once you know how the layers shift, you can explain why two crystals can have the same local coordination but different overall geometry. That is a useful skill in homework problems that ask you to identify a structure from a diagram, compare lattice types, or explain why one solid is denser than another.
If you are reading a problem about ionic radii, metallic bonding, or crystal stability, close packing is often the first structure idea to check. It turns a 3D picture into a pattern you can reason through instead of memorizing one solid at a time.
Keep studying Inorganic Chemistry II Unit 6
Visual cheatsheet
view galleryCoordination number
Close packing gives a coordination number of 12, which means each sphere has 12 nearest neighbors. That number is a quick check that you are dealing with a densely packed lattice rather than a looser arrangement. In crystal structure questions, coordination number often comes right after you identify whether the solid is FCC, HCP, or something more open.
Unit cell
The unit cell is the repeating 3D chunk that builds the whole crystal, and close packing determines what that chunk looks like. In FCC, the close-packed layers stack into a cubic unit cell, while HCP uses a hexagonal unit cell. If you understand the stacking sequence, the unit cell stops feeling like a separate idea and becomes the shape of the packing pattern.
Crystalline structure
Close packing is one way to describe a crystalline structure, especially for metals and some ionic solids. The term helps you move from a particle picture to a repeating lattice with predictable geometry. When a question asks why one solid is denser or more stable than another, crystal structure is usually the bigger framework, and close packing is the specific arrangement inside it.
simple cubic packing
Simple cubic packing is the easiest structure to draw, but it is much less efficient than close packing. It has fewer nearest neighbors and far more empty space, which is why it is not the standard model for dense solids. Comparing simple cubic to close packing is a good way to see why FCC and HCP are treated as the dense limit in solid-state chemistry.
A quiz question might show you a sphere diagram and ask you to identify the packing pattern, count nearest neighbors, or decide whether the structure is FCC or HCP. A problem set may ask you to compare packing efficiency or explain why one solid has a higher density than another.
You may also have to connect the visual structure to the unit cell or to the type of solid. For example, if a lattice diagram shows ABAB stacking, you should recognize HCP. If it shows ABCABC stacking, you should identify FCC or cubic close packing. In written responses, a strong answer does more than name the pattern, it ties the packing to coordination number, unit cell shape, and a property like density or stability.
Simple cubic packing can look similar at first because it also uses spheres in a repeating lattice, but it is much less efficient than close packing. Close packing maximizes space usage with 12 nearest neighbors and about 74 percent packing efficiency, while simple cubic leaves much more empty space. If a structure is described as dense or maximally packed, it is not simple cubic.
Close packing is the densest sphere arrangement used to describe many solid structures in Inorganic Chemistry II.
FCC and HCP are the two main close-packed patterns, and both have a coordination number of 12.
The difference between FCC and HCP comes from the stacking sequence, not from how tightly each layer packs.
Close packing helps you predict density, unit cell shape, and the kinds of solids that form compact lattices.
If a crystal is not close-packed, it usually means bonding, molecular shape, or lattice constraints are forcing a less efficient arrangement.
Close packing is the most efficient way atoms or ions arrange themselves in a crystal lattice. It is the dense sphere model behind FCC and HCP structures, where each particle touches 12 nearest neighbors. In solid-state chemistry, it is a shortcut for thinking about density and coordination.
Both are close-packed and both have 74 percent packing efficiency and coordination number 12. The difference is the stacking order: HCP follows ABAB, while FCC follows ABCABC. That changes the shape of the unit cell, even though the local packing is equally dense.
Look for a dense sphere arrangement with each particle surrounded by 12 neighbors and layers that nest into the gaps of the layer below. In diagrams, FCC and HCP are the classic close-packed examples. If the structure is much more open, like simple cubic, it is not close packed.
Many metals use close-packed lattices because metallic bonding works well in dense arrangements. Ionic solids can also build structures around close-packed layers, depending on ion size and charge balance. The packing pattern helps explain density, stability, and which lattice geometry a solid adopts.