Читать книгу Essentials of Nuclear Medicine Physics, Instrumentation, and Radiation Biology - Rachel A. Powsner - Страница 16

By adding a third dimension to our model of the atom, we can depict the electron orbits as the surfaces of spheres (called shells) to suggest that, unlike the planets orbiting the sun, electrons are not confined to a circular orbit lying in a single plane but may be more widely distributed (Figure 1.5). Although it is convenient for us to talk about distances and diameters of the shells, distance on the atomic scale does not have quite the same meaning it does with everyday objects. The most significant characteristic of a shell is its energy level. The “closer” an electron is to the nucleus, the more tightly it is bound to the nucleus. In saying this, we mean that more work (energy) is required to remove an inner‐shell electron than an outer one. The energy that must be put into the atom to separate an electron is called the electron binding energy. It is usually expressed in electron volts (eV). The electron binding energy varies from a few thousand electron volts (keV) for inner‐shell electrons to just a few eV for the less tightly bound outer‐shell electrons.