General Properties of Solids

In a solid, the attractive forces between atoms, ions, or molecules are strong enough to hold them
rigidly together; thus, the particles’ only motion is vibration about fixed positions, and the kinetic
energy of solids is predominantly vibrational energy. As a result, solids have definite shapes and
volumes.

CRYSTALLINE SOLIDS

A solid may be crystalline or amorphous. A crystalline solid, such as NaCl, possesses an ordered
structure; its atoms exist in a specific three-dimensional geometric arrangement with repeating
patterns of atoms, ions, or molecules. Because of this long-range order, crystalline solids have highly
regular shapes.

The repeating units of crystals are represented by the unit cell, the smallest structural unit that
contains all the information about the spatial arrangement of the particles. Unit cells can be
thought of as building blocks of crystalline solids: Repeating them in all directions will reproduce the
crystal. There are many types of unit cells, differing in geometry, size, etc. Below are the structures
of the three cubic unit cells: simple cubic, body-centered cubic, and face-centered cubic.

The structure of crystalline solids can be determined using the technique of X-ray crystallography.
When electromagnetic radiation (of which X-ray is a type) passes through matter, it interacts with
the electrons and is scattered in different directions. If the matter is made up of a regularly spaced
array of structural units (as is the case for crystals), the X-ray is scattered in such a way that it
produces a characteristic pattern of spots. This phenomenon is known as diffraction. The