The Nucleus Came Next

Transcription

1 The Nucleus Came Next Ernest Rutherford The New Zealand born British chemist and physicist who became known as the father of nuclear physics. He discovered the atomic nucleus, and thereby pioneered the planetary model of he atom, which later evolved into the Bohr model or orbital model of the atom. He was awarded the Nobel Prize in Chemistry in He is widely credited as splitting the atom in 1917 and leading the first experiment to "split the nucleus" in a controlled manner in During the investigation of radioactivity he coined the terms alpha and beta in 1899.

2 Some Background Info on Radiation Symbol Alpha Beta Gamma Mass 4 amu Approx. equal to an electron None Charge Penetration power

3 Gold Foil Experiment (Rutherford Scattering Experiment) The Concept:

4 The Setup

5 Simulation of Rutherford s Experiment (click to run flash) The experiment was actually done by Hans Geiger and Ernest Marsden in 1909, under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester. (Geiger and Marsden were students of Rutherford s.) They measured the deflection of alpha particles (helium ions with a positive charge) directed normally onto a sheet of very thin gold foil. The gold foil was surrounded by a circular sheet of Zinc Sulfide (ZnS) which was used as an indicator. The ZnS sheet would light up when hit with alpha particles. Under the prevailing plum pudding model, the alpha particles should all have been deflected by, at most, a few degrees. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees. From this observation Rutherford was able to observe and deduct the structure of the atom:

6 It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge. Rutherford suggested that a large amount of the atom's charge and mass is instead concentrated into a very physically-small region, giving it a very high electric field. Outside of this "central charge" (later termed the nucleus), he proposed that the atom was mostly empty space. Rutherford was unable to say from the experiment whether the nuclear charge was positive or negative.

7 Rutherford was able to calculate that the radius of his gold central charge would need to be less than 3.4 x meters (the modern value is only about a fifth of this). This was in a gold atom known to be meters or so in radius a very surprising finding, as it implied a strong central charge less than 1/3000th of the diameter of the atom. Although Rutherford's model of the atom had a number of problems which were only resolved following the development of quantum mechanics, the central conclusion from the Geiger Marsden experiment, the existence of the nucleus, still holds. Video Clip 20.6 MB.flv The Neutron In 1932, English Physicist James Chadwick discovered the neutron. For his efforts, Chadwick received the Nobel Prize in How did he do it?

8 You do not need to know this for Mr. Scott s Chemistry course. But if you really want to know, here s how: He performed tests on a new type of radiation which had been baffling physicists for years, and which had previously been mistaken for gamma rays (a form of radiation consisting of highenergy photons). The test, to simplify as much as possible, went like this: A sample of Beryllium was bombarded with alpha particles (another type of naturally occurring radiation which are technically just ionized helium nuclei), which causes it to emit this mysterious radiation. It was then discovered by Irene Joliot- Curie (daughter of Marie and Pierre Curie) and her husband Frederic Joliot-Curie that this radiation, upon striking a proton-rich surface (paraffin was the preferred example), would discharge some of the protons, which could then be detected using a Geiger counter (a device that measures radiation). This was the premise, and from here, Chadwick simply had to play detective and put all the pieces of the puzzle together. For instance, he

9 could tell that the mysterious radiation in question was neutral due to the fact that it was not affected by proximity to a magnetic field, and, unlike standard gamma radiation, did not invoke the photoelectric effect (when photons, such as gamma rays, strike certain surfaces, they discharge electrons, which can be simply measured), but rather discharged protons, which meant that the particles had to be more massive than previously expected. In the end, Chadwick finally solved the puzzle and officially discovered the neutron in 1932.