TABLE OF CONTENTS The TEM and SEM techniques described in earlier chapters yield valuable information about the external or internal structure of a specimen, but little about its chemical composition. Some of the phenomena involved (diffracted electrons in TEM, BSE in the SEM) depend on the local atomic number Z, but not to an extent that would enable us to distinguish between adjacent elements in the periodic table. For that purpose, we need a signal that is highly Z-specific; for example, an effect that involves the electron-shell structure of an atom. Clearly, the latter is element-specific because it determines the position of each element in the periodic table.
A scientific model provides a means of accounting for the properties of an object, preferably using familiar concepts. To understand the electron-shell structure of an atom, we will use the semi-classical description of an atom given by Niels Bohr in 1913. Bohr's concept resembles the Rutherford planetary model insofar as it assumes the atom to consist of a central nucleus (charge = + Ze) surrounded by electrons that behave as particles (charge -e and mass m). The attractive Coulomb force exerted on an electron (by the nucleus) supplies the centripetal force necessary to keep the electron in a circular orbit of radius r.
| (6.1) |  |
Here, K = 1/(4 ? ? 0) is the Coulomb constant and v is the tangential speed of the electron. The...
