Determining Crystal Structure by X-Ray Diffraction

You can relate the resulting pattern produced by the diffraction to the structure of the crystal. You can determine the type of unit cell and its size if the solid is composed of molecules, you can determine the position of each atom in the molecule. 

Max von Lane, a German physicist, was the first to suggest the use of x rays for the determination of crystal structure. Soon afterward, in 1913, the British physicists WiUiam Bragg and his son Lawrence developed the method on which modem crystal-structure determination is based. They realized that the atoms in a crystal form reflecting planes for x rays, and from this idea they derived the fundamental equation of crystal-structure determination. 

The Bragg equation relates the wavelength of x rays, A, to the distance between atomic planes, d, and the angle of reflection, 0. Note that reflections occur at several angles, corresponding to different integer values of n. 

With the development of electronic x-ray detectors and minicomputers, x-ray diffraction has become automated, so that the time and effort of determining the structure of a molecule have been substantially reduced. Now frequently the most difficult task is preparing a suitable crystal. The crystal should be several tenths of a millimeter in each dimension and without significant defects. Such crystals of protein molecules, for example, can be especially difficult to prepare. 

Water is the only liquid substance (other than petroleum) to be found on earth in significant amounts. This liquid is also readily convertible nnder conditions on earth to the solid and gaseons forms. Water has several un-usnal properties that set it apart from other snbstances. For example, its solid phase, ice, is less dense than liqnid water, whereas for most substances the solid phase is more dense than the liqnid. In addition, water has an nnnsnally large heat capacity. These are some of the properties that are important 

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