X-Rays in Chemistry
Q1. Why are X-rays used and not the visible light for the crystal structure characterization of chemical compounds?
A1. X-rays and visible light are both the part of electromagnetic radiation however, X-rays have very high energy and very small wavelength compared to visible lights which has very low energy and very high wavelength. The small wavelength of X-rays is comparable to the inter-atomic distances in the crystal as against the large wavelength of the visible light.
Since the main focus of structural characterization is to be able to see the spatial arrangement of atoms in a crystal structure therefore, X-rays are more suitable compared to the visible light.
The follwing animation should help you understand the difference in the wavelength between X-rays and visible light and their capability to help see the crystal structure.
Q2. What are the various applications of X-rays in Chemistry?
A2. X-rays find various applications based on the phenomena it exhibits due to its interaction with a specimen. These phenomena can be classified into following three categories:
(a) Absorption: when an X-ray beam strikes the specimen and gets absorbed. This is often used for medical purposes to know about the bone fractures, etc. by passing low-energy X-ray through the affected body part.
(b) Scattering: is the basis of materials analysis that involves characterization by means of atomic arrangement in the crystal lattice. Each crystal is made up of a regular, ordered array of atoms in three dimension; each atom in this array has certain electron density based on the atomic number of that particular atom. The incident X-ray beam is scattered by these electron densities and generates a diffraction pattern that can be used to determine the crystal structure of a given chemical.
(c) Fluorescence: is a phenomena that occurs when a specimen is irradiated with high energy X-rays fall and the innermost electrons are ejected. The resulting gap is filled by the outer shell electrons falling down to fill the generated gap. The energy of outer shell electron must match the energy of inner shell electron for it to be able to occupy the inner shell. The excess energy is released in the form of fluorescence. Since each atom in the periodic table releases a characteristic amount of energy, it is possible to perform a qualitatitve and quantitative elemental analysis of any specimen.