Crystallography

From ArticleWorld

Crystallography is the science that analyzes solids to determine the arrangement of the atoms in them. In other words it is the study of crystal structures. Older methods of studying crystals include the measuring of angles of crystal faces to determine the geometry of the crystal and establish symmetry. This method plots the position in 3D space of each face and determines symmetry in the final plot.

Technology

New technologies such as X-ray diffraction have made this process less tedious. The analysis is now dependent on the diffraction pattern obtained after the sample is exposed to a beam of some sort. The beam used may be X-rays, electrons or neutrons. These all act in different ways making them suitable for a variety of studies. X-rays interact with the spatial distribution of the valence electrons, electrons interact with the total charge distribution of the entire molecule and neutrons are scattered by the atomic nuclei and the magnetic fields.

Theory

In crystallography it is necessary to use short wavelength beams rather than the longer wavelength beams as the atomic bond lengths are short in comparison. This means that the use of visible light is not appropriate but that X-rays may be used. The use of X-rays means that focusing an image is made difficult. This is where crystals play a large role. Due to their ordered structure crystals provide a means to analyze other solids.

Techniques

Materials such as DNA do not occur as crystals and as such require other techniques to bring them into the crystal form in order to be studied by crystallography. These molecules are placed in a solution are allowed to slowly crystallize over a period of days, weeks or months by vapor diffusion. This method uses a drop of solution which is composed of the molecule, buffer and precipitants sealed in a container with a reservoir of a hygroscopic solution. It is seen that the water in the drop diffuses and this concentrates the solution and precipitates the molecule over time. When the crystal is obtained the analysis of the structure can then take place.