Quantum Physics

904 Chapter 28 Atomic PhysicsElectronFigure 28.1 Thomson’s modelof the atom, with the electronsembedded inside the positive chargelike seeds in a watermelon.SIR JOSEPH JOHN THOMSON,English Physicist (1856 – 1940)Thomson, usually considered the discovererof the electron, opened up the field ofsubatomic particle physics with his extensivework on the deflection of cathoderays (electrons) in an electric field. Hereceived the 1906 Nobel prize for hisdiscovery of the electron.Stock Montage, Inc.atoms. J. J. Thomson (1856–1940) suggested a model of the atom as a volume ofpositive charge with electrons embedded throughout the volume, much like theseeds in a watermelon (Fig. 28.1).In 1911 Ernest Rutherford (1871–1937) and his students Hans Geiger andErnest Marsden performed a critical experiment showing that Thomson’s modelcouldn’t be correct. In this experiment, a beam of positively charged alpha particleswas projected against a thin metal foil, as in Figure 28.2a. The results of theexperiment were astounding. Most of the alpha particles passed through the foil asif it were empty space, but a few particles deflected from their original direction oftravel were scattered through large angles. Some particles were even deflectedbackwards, reversing their direction of travel. When Geiger informed Rutherfordof these results, Rutherford wrote, “It was quite the most incredible event that hasever happened to me in my life. It was almost as incredible as if you fired a 15-inchshell at a piece of tissue paper and it came back and hit you.”Such large deflections were not expected on the basis of Thomson’s model. Accordingto that model, a positively charged alpha particle would never come closeenough to a large positive charge to cause any large-angle deflections. Rutherfordexplained these astounding results by assuming that the positive charge in an atomwas concentrated in a region that was small relative to the size of the atom. Hecalled this concentration of positive charge the nucleus of the atom. Any electronsbelonging to the atom were assumed to be in the relatively large volume outsidethe nucleus. In order to explain why electrons in this outer region of the atomwere not pulled into the nucleus, Rutherford viewed them as moving in orbitsabout the positively charged nucleus in the same way that planets orbit the Sun, asshown in Figure 28.2b. Alpha particles themselves were later identified as the nucleiof helium atoms.There are two basic difficulties with Rutherford’s planetary model. First, an atomemits certain discrete characteristic frequencies of electromagnetic radiation and noothers; the Rutherford model is unable to explain this phenomenon. Second, theelectrons in Rutherford’s model undergo a centripetal acceleration. According toMaxwell’s theory of electromagnetism, centripetally accelerated charges revolvingwith frequency f should radiate electromagnetic waves of the same frequency.Unfortunately, this classical model leads to disaster when applied to the atom. As theelectron radiates energy, the radius of its orbit steadily decreases and its frequency ofrevolution increases. This leads to an ever-increasing frequency of emitted radiationand a rapid collapse of the atom as the electron spirals into the nucleus.28.2 ATOMIC SPECTRAThe hydrogen atom is the simplest atomic system and an especially important oneto understand. Much of what we know about the hydrogen atom (which consists ofone proton and one electron) can be extended directly to other single-electronViewingscreen–Figure 28.2 (a) Geiger andMarsden’s technique for observingthe scattering of alpha particlesfrom a thin foil target. The sourceis a naturally occurring radioactivesubstance, such as radium.(b) Rutherford’s planetary modelof the atom.SourceLeadscreen(a)Target–+(b)