Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

The atomic mass unit (amu) is defined as l/12thof the mass of an atom of 12c.It can be shown that 1 amu is equivalent to 931 MeV of energy.The atom is the smallest division of an element in which the chemical identity ofthe element is maintained. The atom is composed of an extremely dense positivelycharged nucleus containing protons and neutrons and an extranuclear cloud oflightnegatively charged electrons. In its nonionized state, an atom is electrically neutralbecause the number of protons equals the number of electrons. The radius of anatom is approximately 10- 10 m, whereas that of the nucleus is only about 10- 14 m.Thus, the atom is largely unoccupied space, in which the volume of the nucleus isonly 10- 12 (a millionth of a millionth) the volume of the atom. If the empty spacein an atom could be removed, a cubic centimeter of protons would have a mass ofapproximately 4 million metric tons.Electronic StructureElectron Orbits and Electron Binding EnergyIn the Bohr model of the atom (proposed by Niels Bohr in 1913) electrons orbitaround a dense positively charged nucleus at fixed distances. Each electron occupiesa discrete energy state in a given electron shell. These electron shells are assigned theletters K, L, M, N, ..., with K denoting the innermost shell. The shells are alsoassigned the quantum numbers 1, 2, 3, 4, ... , with the quantum number 1 designatingthe K shell. Each shell can contain a maximum number of electrons given by(2n 2 ), where n is the quantum number of the shell. Thus, the K shell (n = 1) canonly hold 2 electrons, the L shell (n = 2) can hold 2(2)2 or 8 electrons, and so on,as shown in Fig. 2-4.8K""\\\\ \ \\L M N 0 P Q) ))))).-/.:/ ,I:: :.:: :/ /'Quantum # 2 3 4 5 6 7Maximum 2 8 18 32 50 72 98ElectronFIGURE 2-4. Electron shell des-Capacity ignations and orbital filling rules.