Quantum Physics

“Neon lights,” commonly used inadvertising signs, consist of thin glasstubes filled with various gases, suchas neon and helium. The gas atomsare excited to higher energy levels byelectric discharge through the tube.When the electrons in these excitedlevels return to lower energy levels,the atoms emit light having awavelength (color) that depends onthe type of gas in the tube. Forexample, a tube filled with neonproduces a red-orange color, whilehelium produces pink.Atomic PhysicsA large portion of this chapter concerns the hydrogen atom. Although the hydrogen atom isthe simplest atomic system, it’s especially important for several reasons:• The quantum numbers used to characterize the allowed states of hydrogen can also beused to describe (approximately) the allowed states of more complex atoms. This enablesus to understand the periodic table of the elements, one of the greatest triumphs of quantummechanics.• The hydrogen atom is an ideal system for performing precise comparisons of theory withexperiment and for improving our overall understanding of atomic structure.• Much of what is learned about the hydrogen atom with its single electron can be extendedto such single-electron ions as He and Li 2 .In this chapter we first discuss the Bohr model of hydrogen, which helps us understand manyfeatures of that element but fails to explain finer details of atomic structure. Next we examinethe hydrogen atom from the viewpoint of quantum mechanics and the quantum numbersused to characterize various atomic states. Quantum numbers aren’t mere mathematical abstractions:they have physical significance, such as the role they play in the effect of a magneticfield on certain quantum states. The fact that no two electrons in an atom can have thesame set of quantum numbers—the Pauli exclusion principle—is extremely important in understandingthe properties of complex atoms and the arrangement of elements in the periodictable. Finally, we apply our knowledge of atomic structure to describe the mechanismsinvolved in the production of x-rays, the operation of a laser, and the behavior of solid-statedevices such as diodes and transistors.Dembinsky Photo AssociatesCHAPTER28O U T L I N E28.1 Early Models of the Atom28.2 Atomic Spectra28.3 The Bohr Theoryof Hydrogen28.4 Modification of the BohrTheory28.5 De Broglie Waves and theHydrogen Atom28.6 Quantum Mechanics andthe Hydrogen Atom28.7 The Spin MagneticQuantum Number28.8 Electron Clouds28.9 The Exclusion Principleand the Periodic Table28.10 Characteristic X-Rays28.11 Atomic Transitions28.12 Lasers and Holography28.13 Energy Bands in Solids28.14 Semiconductor Devices28.1 EARLY MODELS OF THE ATOMThe model of the atom in the days of Newton was a tiny, hard, indestructiblesphere. Although this model was a good basis for the kinetic theory of gases, newmodels had to be devised when later experiments revealed the electronic nature of903