The Gold Foil Experiment

For example, it was known that carbon has an atomic weight of 12 and that six electrons could be stripped away to form an ion with a charge of +6. To account for this information, the nuclear model would suggest that a nucleus of carbon contains 12 protons (since protons would be responsible for the mass) and six electrons, giving the nucleus a net charge of 

Furthermore, Rutherford was able to measure the relative size of the nucleus and the space occupied by the six outer electrons (which would determine the size of the atom itself). He concluded that the diameter of the atom was about 10,000 times larger than the diameter of the nucleus. Since the diameter of a gold atom is on the order of 10 ° m, that would make the diameter of the nucleus on the order of about lO m. 

In ensuing years, scientists like Rutherford began to suspect that atoms contained a third, neutral particle, but the discovery of the neutron was two decades away. Once the neutron was discovered, the nuclear model became one in which the nucleus contained protons and neutrons, but no electrons. The number of protons corresponded to the charge on the nucleus, and the sum of the numbers of protons and neutrons determined the mass of the atom. The electrons were located only in the space outside the nucleus, the number of electrons in a neutral atom being equal to the number of protons. Eventually, the concept of the atomic number of an element was developed to correspond to the number of protons in an atom. The concept of the mass number of a specific isotope was developed to correspond to the sum of the numbers of protons and neutrons. 

For many purposes in chemistry, the nuclear model of the atom is still a perfectly sufficient model for picturing atoms. It explains atomic numbers and atomic weights, allows for different isotopes of elements as well as different ions, and is all that is necessary to study mass relationships in chemical reactions. Physicists and chemists, however, were not satisfied just to say that the electrons are somewhere in the region of space outside the nucleus. A more detailed understanding of how the electrons are distributed was both the next subject of their investigations and the subject of our next chapter. 

The work being described in this chapter mostly took place from about 1913 to 1926. The development of our modem model of the atom—the quantum mechanical model—is a remarkable achievement in the history of science. Much of twenty-first century science and technology depends on the properties of atoms and molecules as described in the quantum mechanical model of the atom. 

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Read about the gold foil experiment in your textbook. Describe the plum-pudding atomic model. How did the gold foil experiment show the plumpudding model to be in error Describe the nuclear atomic model that replaced the plumpudding model. 

To Rutherfords surprise, what was the fate of a tiny fraction of alpha particles in the gold-foil experiment ... 

E) All of the above are correct regarding the gold foil experiment. 

The positively charged particles that repelled the alpha particles in the gold foil experiments and that compose the nucleus of an atom are called protons. The charge of a proton was calculated to be exactly equal in magnitude but opposite in sign to the charge of an electron. Later experiments showed that the proton s mass is almost 2000 times the mass of an electron. 

How did the results of the gold foil experiment lead Rutherford to recognize the existence of atomic nuclei ... 

Rutherford concluded that the plum pudding model was incorrect because it could not explain the results of the gold foil experiment. He set out to develop a new atomic model based upon his findings. Considering the properties of the alpha particles and the electrons, and the frequency of the deflections, he calculated that an atom consisted mostly of empty space through which the electrons move. He also concluded that there was a tiny, dense region, which he called the nucleus, centrally located within the atom that contained all of an atom s positive charge and virtually all of its mass. Because the nucleus occupies such a small space and contains most of an atom s mass, it is incredibly dense. Just how dense If a nucleus were the size of the dot in the exclamation point at the end of this sentence, its mass would be approximately as much as that of 70 automobiles ... 

Rutherford s nuclear model of the atom explains the results of the gold foil experiment. Most alpha particles pass straight through, being only slightly deflected by electrons, if at all. The strong force of repulsion between the positive nucleus and the positive alpha particles causes the large deflections. 

With the gold foil experiment, Ernest Rutherford determines properties of the nucleus, including charge, relative size, and density. 

Interactive Figure To see an animation of the gold foil experiment, visit qlencoe.com. 

FIGURE 3.3 Rutherford s interpretation of the gold foil experiment done by Geiger and Marsden. Each circle represents an atom and the dots represent their nuclei. The gold foil was about 1000 atoms thick. 

A beam of alpha particles was focused through a slit in a circular screen that had a phosphorescent coating of ZnS on its interior surface. When an energetic alpha particle struck the phosphorescent screen, it would be observed as a flash of light. In the center of the apparatus was mounted a very thin piece of metal foil (although it is often referred to as the gold foil experiment, it was in fact a piece of platinum foil, not gold, which was used). While the majority of alpha particles struck the screen... 

Atomic view of the gold foil experiment. If the plum pudding model of the atom were accurate, a beam of massive alpha particles would penetrate right through the atom with little or no deflections (a). The observation that some of the alpha particles were deflected backward implied that the positive charge in the atom must be confined to a highly dense region inside the atom known as the nudeus (b). [Blatt Communications.]... 

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