How are we to learn about the behavior of systems of the atomic scale, eight orders of magnitude smaller than things familiar in out experience? How is it possible, with instruments of a size that we can manipulate with our hands, to determine the molecular arrangement. If the molecular can be persuaded to crystallize into a periodic array, a regular repeating stacking of the molecule, then the x-ray diffraction experiment can do the job. It is based on simple physical principles which are illustrated by "bravais". In "bravais", we not focus on complex molecules, but on simple two-dimensional crystals containing

one or two atoms per molecule. The size and shape of the unit cell define possible directions, if any, for diffracted beams; the cell content or basic determines the intensity of any diffracted beams. A two dimensional crystal structure may be defined in terms of a unit cell, a parallelogram with sides of length a1 and a2 and an interior angle; and a basic giving the positions of atoms within the cell. The full crystal is generated by tiling define the crystal lattice or Bravais lattice associated with the crystal structure. Thus the definition of a crystal structure is a two step process. First, we define a Bravais lattice in terms of the parameters of the unit cell. This lattice has the same translational symmetry as the crystal structure. Then, defining the position of the atoms within the cell, the basic, completes the description.

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