Solids are divided into two categories: crystalline solids (solids with regular arrangement of their components), and amorphous solids (solids with disorder in the structure). Positions of the components in a solid can be represented by a lattice, a three-dimensional point that designates the positions of the components that make up the substance.

Types of Crystalline Solids

Many types of crystalline solids exist. The important types are ionic solids (i.e. sodium chloride), molecular solids (i.e. sucrose), atomic solids (i.e. diamond), and metallic solids (i.e. copper).

The classification of a solid is determined by what occupies its lattice points:

The Unit Cell

  • Smallest unit of a solid
  • Face centered cubic (fcc) and body centered cubic (bcc)
    • fcc: same length on each side, particle at each corner, and a particle on each face
    • bbc: similar to ffc except instead of particles on each face, it has a particle on the center of the cube
  • Unit cell in ionic solids
    • Ratio of ions in the unit cell is the same as the ratio in the overall structure
    • Size of unit cell depends on the size of the ions and their arrangement
  • If repeated, the unit cells could generate the entire solid

X-Ray Analysis of Solids

The structures of crystalline solids are most commonly determined by X-ray diffraction, directing X-rays of a single wavelength at a crystal and obtaining a diffraction pattern from which interatomic spaces can be determined. Diffraction is due to constructive interference when the waves of parallel beams are in phase and to destructive interference when the waves are out of phase. When X-rays of a single wavelength are directed at a crystal, a diffraction pattern is obtained.

In order to carry out an X-ray analysis of crystals, scientists use diffractometers, which rotate the crystal and scatter the X ray from various planes of the atoms in the crystal. The data produced by the scattering of the rays are then analyzed by computer, allowing for the classification of complex structures. For example, through diffractometers, the structure of enzymes have been determined, allowing biochemists to understand their multiple functions in the body. Using X-ray diffraction also gives us a new method in gathering data on bond lenghts and angles for our models of molecular geometry.


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