Imagine looking at the shadow of a chandelier on a white wall. Using the shadow as your only source of information, you have to describe the structure of the chandelier. This includes the number of pendants, their relative sizes and positions. This is comparable to the problem of the X-ray crystallographer using photographic images to understand the internal structures of materials. “The properties of the metals must depend on the properties of the individual atoms and on the atomic arrangement" wrote Sir William Bragg in 'Concerning the Nature of Things' published in 1925. The Braggs, father and son, were responsible for Bragg's Law which describes the way X-rays interact with crystals. Visible light cannot penetrate most crystals but X-rays of shorter wavelength can do so. The use of X-ray crystallography has made possible an understanding of the formation and properties of crystalline materials. Bragg's book of 1925 is consciously named after that of the Roman philosopher Lucretius (c95-55BC) in which he described his own atomic theory.
Crystals have a regular geometrical form known as a lattice. The lattice is produced by the systematic arrangement of particles in the crystal. The particles may be atoms, ions or molecules. The basic pattern is repeated in three dimensions to build up the complete crystal.
In a pure metal all the atoms are identical. Layers of atoms can slide past one another when subjected to a shear force. Crystals are rarely perfect at the atomic scale. Sometimes atoms are missing from the structure, these are called vacancies. Another imperfection occurs when crystal planes do not match exactly, these are dislocations. Dislocations can move through the crystal, they separate an unslipped region from a slipped one. If you want to adjust one carpet that is resting on another one it is very difficult to drag the whole carpet along at once. If you ruck up the top carpet, the ruck can be moved quite easily, like a dislocation moving through a crystal. Dislocations make metals ductile; they can be drawn into wires.
Perfect single crystals have special uses such as silicon crystals for semiconductors and titanium in the turbine blades of aeroengines. All metals are aggregates of crystals, they are polycrystalline. If the crystal grains are very small the metal may exhibit superplasticity. A superplastic metal can be stretched several hundred per cent without breaking. With smaller grains still the material becomes a glassy metal. The only metal that is liquid at room temperature is mercury, the famous quicksilver.