A FCC unit cell contains four atoms: one-eighth of an atom at each of the eight corners ( atom from the corners) and one-half of an atom on each of the six faces ( atoms from the faces). This arrangement is called a face-centered cubic (FCC) solid. Many other metals, such as aluminum, copper, and lead, crystallize in an arrangement that has a cubic unit cell with atoms at all of the corners and at the centers of each face, as illustrated in Figure 7. (Elements or compounds that crystallize with the same structure are said to be isomorphous.) Isomorphous metals with a BCC structure include K, Ba, Cr, Mo, W, and Fe at room temperature. Each atom touches four atoms in the layer above it and four atoms in the layer below it.Ītoms in BCC arrangements are much more efficiently packed than in a simple cubic structure, occupying about 68% of the total volume. In a body-centered cubic structure, atoms in a specific layer do not touch each other. The entire structure then consists of this unit cell repeating in three dimensions, as illustrated in Figure 1.įigure 6. The unit cell consists of lattice points that represent the locations of atoms or ions. The structure of a crystalline solid, whether a metal or not, is best described by considering its simplest repeating unit, which is referred to as its unit cell. We will explore the similarities and differences of four of the most common metal crystal geometries in the sections that follow. The different properties of one metal compared to another partially depend on the sizes of their atoms and the specifics of their spatial arrangements.
Some of the properties of metals in general, such as their malleability and ductility, are largely due to having identical atoms arranged in a regular pattern. A pure metal is a crystalline solid with metal atoms packed closely together in a repeating pattern. We will begin our discussion of crystalline solids by considering elemental metals, which are relatively simple because each contains only one type of atom. In this module, we will explore some of the details about the structures of metallic and ionic crystalline solids, and learn how these structures are determined experimentally.
The regular arrangement at an atomic level is often reflected at a macroscopic level.
Most solids form with a regular arrangement of their particles because the overall attractive interactions between particles are maximized, and the total intermolecular energy is minimized, when the particles pack in the most efficient manner. Over 90% of naturally occurring and man-made solids are crystalline. Explain the use of X-ray diffraction measurements in determining crystalline structures.Compute ionic radii using unit cell dimensions.Describe the arrangement of atoms and ions in crystalline structures.