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Transcript
The Rutherford Experiment and Hit the Penny The Thomson Model With the growing evidence that the atom is made of even smaller particles, in the early 1900s J. J. Thomson reasoned that because electrons comprise only a very small fraction of the mass of an atom (they are very small), they probably made up an equally small fraction of the atom's size. He proposed that the atom consisted of a positive sphere of matter with electrons in it, as shown. This model became known as the "plum-pudding" model, after the name of a traditional English dessert. J. J. Thomson's "plum-pudding" model of the atom. He pictured the small electrons to be embedded in the atom much like raisins in a pudding or like seeds in a watermelon. Ernest Rutherford proved this model wrong. The Rutherford Experiment In 1910 Rutherford and his coworkers performed an experiment that led to the downfall of Thomson's model. Rutherford was studying how alpha-particles were scattered as they passed through the atoms of thin gold foil. Alpha particles are small but dense particles. Think of them as tiny hard bullets. He found that most alpha particles went straight through the gold atoms. He expected this because the Thomson model said most of the atom was just a “pudding” of positive matter, nothing “hard” for an alpha-particle to hit. However, Rutherford was in for a surprise! In Rutherford's own words: “I remember two or three days later my assistant coming to me in great excitement and saying, "We have been able to get some alpha particles coming backwards." . . . It was quite the most incredible event that has ever Most alpha particles go happened to me in my life. It was almost straight as if you fired a bullet into a piece of through tissue paper and it came back and hit you.” Rutherford and his coworkers observed that almost all of the alpha particles passed directly through the foil without deflection. However, a few were deflected, some even bouncing back in the direction from which they had come By 1911 Rutherford was able to explain these observations; he thought that Beam of alpha particles Radioactive source of alpha particles A few alpha particles hit the nucleus and bounce back 1. there was a small, dense nucleus with most of the mass of the atom, and all of its positive charge, (because only a small number of alpha particles bounced back). 2. Most of the total volume of the atom is empty space in which electrons move around the nucleus. Because in the alpha-particle scattering experiment, most alpha particles passed directly through the gold atoms because they do not hit the small nucleus; they merely pass through the empty space of the atom. Occasionally an alpha particle comes into the close vicinity of a gold nucleus, however. The repulsion between the highly charged gold nucleus and the alpha particle is strong enough to cause the alpha particle to bounce back. The Hit the Penny Simulation of the Rutherford Experiment In Hit the Penny, we dropped pencils (model alpha particles) onto a model atom consisting of a piece of paper with a penny on it for a nucleus. Most of the time, we missed the penny, just as most of Rutherford’s alpha particles missed the nucleus. a. Hit the penny: c. b Penny . Paper . d