Face-Centered
Cubic (FCC) is one of the three
kinds of cubic
lattice structures,
Simple Cubic and
Body-Centered Cubic being the others. It is one of the fourteen
Bravais lattices.
FCC lattices, along with
HCP lattices, are the most
efficient uses of
space nature has in terms of
solid state chemistry. Each unit cell of an FCC
solid is considered to have 4 molecules.
In each unit cell of an FCC solid, there are molecules on each of the eight corners of the cube. In addition to this, there is a molecule on each of the six faces of the cube. This is why Face-Centered Cubic is called what it is: the extra molecules are on the faces of each unit cell, rather than wholly contained within the body, as in a body-centered cubic structure.
As stated before, the FCC structure is tied for the most efficient Bravais lattice. As a result, many elements naturally adopt FCC: gold, copper, silver, aluminum, cesium, nickel and lead are just a few.
Let's see why each unit cell is considered to have 4 molecules. As in SC and BCC, each molecule on the 8 corners of the cube are split between 8 unit cells, so they all collectively add up to one. In FCC, there is a molecule on each of the six faces of the cube, which is shared with one other unit cell, which means each one counts as half a molecule. They all add up to three. Together, these account for all four molecules per unit cell.