Properties of solids

Properties of solids

Types of solids

Crystalline solids have atoms, molecules or ions arranged in a regular pattern. Sodium chloride is an example of a crystalline solid. These solids have a fundamental building block called a unit cell. The unit cell consists of a set of points that mark out the positions of atoms, ions or molecules. The locations of these points can generate a complete crystal when stacked or repeated in three dimensions.

Amorphous solids have no regular pattern for molecule, atom or ion locations. Glass and tar are examples of amorphous solids. These types of matrials have NO simple repeated unit cell. The structure is more complicated because they lack the repetitive units.

Examples of unit cells

Cubic unit cells: Simple cubic, body centered cubic, face entered cubic

simple cubic unit cell body centered unit cell face centered unit cell

These three cubic unit cells illustrate the regularity in crystal lattice structures. The repetition of these unit cells produces a total solid.

Number of atoms in a unit cell: SCC

The locations of atoms in the simple cubic unit cell are marked by eight points at the eight corners of the cube. The centers of the atoms are at the corners. Each atom is shared by eight adjacent cubes. One eighth of an atom is inside the cube at each corner. Eight corners times 1/8 results in only one atom actually being inside the cube.

Number of atoms in a unit cell: BCC (illustrated in animation )

Body centered cubic has two atoms in a unit cell. There is one in the center and 1/8 from each of the eight corners. The total adds to two atoms inside the unit cell.

This animation shows a body centered cubic unit cell. The spheres are not shown as touching even though the spheres really would be in contact across the diagonal of the cube. The spheres on the edge do not touch one another.

Number of atoms in a unit cell: FCC

The face centered unit cell has one atom form the corners again and one half from each face. There are six faces so from the faces there are 6 x 1/2 = 3 atoms. The total number of atoms in the unit cell is four.