face-centered cubic
/ fays-SEN-terd KYOO-bik /
Stack oranges in a crate the way a greengrocer does: lay a layer, then nestle the next layer's oranges into the dimples of the one below, again and again. You will have packed them about as tightly as round things can go. If you mark where the orange centers sit and tidy the picture into a cube, you have arrived at the face-centered cubic arrangement.
Face-centered cubic, or fcc, is a cubic structure with an atom at each of the cube's eight corners and one more at the center of each of its six faces — hence face-centered. Counted properly (corners shared eight ways, faces shared two ways), that works out to four atoms per cube. It is one of the close-packed structures: each atom touches twelve nearest neighbours, the most that identical spheres can manage, and the packing fills about 74 percent of space. Its primitive cell is smaller and skewed, but the cube is the easy way to picture it.
Fcc matters because many of our most useful metals — aluminium, copper, silver, gold, nickel, lead — crystallize this way, and the close, slippery planes of atoms make them soft and easy to roll, draw, and shape. A common confusion is to mix it up with body-centered cubic, which has an atom in the cube's center rather than on its faces, packs less densely, and behaves differently. Fcc and hexagonal close-packed are both close-packed and differ only in how the layers are stacked.
A gold ring is fcc gold. The reason gold can be hammered into leaf thin enough to see through is that its close-packed fcc planes slide over one another easily, letting the metal flatten without cracking.
Sliding close-packed planes let fcc gold be beaten into see-through leaf.
Face-centered cubic and cubic close-packed are two names for the very same structure — one names the cube, the other names the stacking of close-packed layers. They are not different things.