20.3 X-Rays
The transitions of electrons in the hydrogen or other light elements result in the emission of spectral lines in the infrared, visible or ultraviolet region of electromagnetic spectrum due to small energy differences in the transition levels. In heavy atoms, the electrons are assumed to be arranged in concentric shells labelled as K, L, M, N, O, etc. The K-shell being closest to the nucleus, the L-shell next, and so on (Fig. 20.3). The inner shell electrons are tightly bound and large amount of energy is required for their displacement from their normal energy levels. After excitation, when an atom returns to its normal state, photons of larger energy are emitted. Thus transition of inner shell electrons in heavy atoms gives rise to the emission of high energy photons or X-rays. These X-rays consist of series of specific wavelengths or frequencies and hence are called characteristic X-rays. The study of characteristic X-rays spectra has played a very important role in the study of atomic structure and the periodic table of elements (Fig. 20.4).
(Fig. 20.3: Concentric energy shells)
(Fig. 20.4: Electromagnetic spectrum)