Wednesday, October 6, 2010

Rutherford's Gold Foil Experiment


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                        Rutherford is best known for his gold foil experiment, which greatly contributed to the model of the atom as we know it today. His experiment introduced a new understanding about the atom, and proved his former teacher’s model incorrect. The gold foil experiment was conducted in 1909 at the University of Manchester by Geiger and Marsden, but his theory of the atom wasn’t complete until 1911 (4). 

Ernest Rutherford had the opportunity to study and research at a variety of colleges. At the colleges, he was able to make many discoveries that would assist him in his gold foil experiment. He studied radioactivity, which led him to discover alpha particles, the main focus in the gold foil experiment. Ernest Rutherford was born on August 30, 1871 in Spring Gold, New Zealand. He obtained his early education through government schools, and at the age of 16, he entered Nelson Collegiate School. In 1889 he was awarded a University scholarship to Canterbury College at the University of New Zealand. He graduated from Canterbury with a double major in mathematics as well as physical science. In the year 1849, he was awarded an 1851 Exhibition Science Scholarship that allowed him to study at Trinity College at the University of Cambridge as a research student. At Trinity College, he worked with J.J. Thomson in the Cavendish Laboratory, who would become a very important figure in the gold foil experiment (1). While working with Thomson, Rutherford studied radiation, which led to his interest in alpha particles. He was offered a job as a chemistry professor to be the Macdonald Chair of Physics at McGill University in Montreal. In 1898, he moved to Canada to take the job, where he continued to work with radioactivity. His research led him to discover that radioactivity was the "spontaneous disintegration of atoms" (2). Due to all the work he did at McGill University, he was awarded the Nobel Prize in Chemistry in 1908. In 1907, he succeeded Arthur Schuster at the University of Manchester, where he began an in-depth study of alpha particles (1). His research led him to discover that alpha particles cause a weak, but discrete flash when they strike a luminescent zinc sulfide screen (4). His research and findings about alpha particles would later prove to be very important in his gold foil experiment.

                                                      Ernest Rutherford (1)

Who was involved in the gold foil experiment?
Rutherford was credited for the gold foil experiment, but two other people played a important role in the experiment as well. Under the supervision of Rutherford, Geiger and Marsden performed the gold foil experiment. Hans Geiger, Rutherford’s partner, had worked with him at the University of Manchester since 1908, and Ernest Marsden was a student at the University of Manchester (4). Although J.J. Thomson wasn’t directly involved with the experiment, he played a key role, serving as the reason the experiment was performed. Rutherford was a former student of Thomson and highly looked up to him. The reason behind the gold foil experiment was to prove Thomson’s “plum pudding model” (5).  

                                                Rutherford and Geiger (2)

J.J Thomson’s “Plum Pudding Model”
The “plum pudding model” was J.J. Thomson’s theory of atomic structure that he developed in 1904 (3). His theory was that all the subatomic particles were spread evenly throughout the atom in one positively charged piece. In other words, negatively charged electrons were floating in a mass of positive charge (4).

Thomson believed the negative electron "plums" floated around in a large mass of positive         "pudding"  (3)

The experiment:
The gold foil experiment was ultimately performed in order to prove Thomson’s “plum pudding model”, although that was not the case. Just the opposite happened, proving the theory incorrect. The experiment consisted mostly of alpha particles and gold foil (5). An alpha particle is a helium nucleus released by radioactive substances (discovered when Rutherford was studying radioactivity). It is a fairly heavy, positively charged particle (6). To begin, polonium was put into a lead box that sent out alpha particles to a thin sheet of gold foil. The foil was then surrounded by a luminescent zinc sulfide screen that served as a backdrop for the alpha particles to appear on (4).  A microscope was placed above the screen so they could easily observe any contact made between the alpha particles and the screen. In order to see the light of the alpha rays more clearly, the experiment was performed in complete darkness. To begin the experiment, they aimed a beam of alpha particles at a piece of gold foil, and then observed the astonishing results (5).

A simple diagram of how the experiment was set up. (4)
            In order to prove Thomson’s “plum pudding model”, the alpha particles were supposed to go straight through the foil. Shockingly, although most alpha particles passed straight through the gold foil, some did not. A small amount of particles were deflected slightly from the straight path by only about one or two degrees. An even smaller amount of particles were reflected off the gold foil at very large angles, and some occasionally bounced directly back at the source (3). The fact that some particles bounced back at large angles was the most surprising factor of the whole experiment. They discovered that 1 in 20,000 particles would bounce back at approximately 90 degrees or more from the parent beam. Rutherford was so amazed by this outcome, he explained the result by saying, “It was as if you fired a 15-inch shell as a sheet of tissue paper and it came back to hit you.” (5).  

            Obviously, Thomson’s “plum pudding model” was proven incorrect. Thomson’s theory said that an atom was made up of empty space, but that couldn’t be correct if the particles had bounced back because they had to have hit something. Rutherford reasoned that the item that was hit must have been very small since the majority of the particles didn’t bounce off of it (5). Alpha particles are very heavy, and have a mass about 8,000 times that of an electron. The alpha particles were traveling at a very high speed when they hit the foil, so a strong force was necessary in order to redirect the alpha particles since they were already so heavy (6). The way and angles in which the alpha particles bounced off the foil indicated that the majority of the mass of an atom was concentrated in one small region, that Rutherford later called the nucleus (3). He reasoned that the nucleus held all the positive charge, while electrons occupied most of the atom’s space. In simpler terms, the atom was made up of mostly empty space, which was why the majority of the alpha particles passed through. The particles that were deflected must have hit something in order for it to bounce off. This item that the particles bounced off of was named the nucleus.

This model describes how some of the alpha particles passed straight through, and how others didn't. As seen in picture (b), some of the alpha rays hit the nucleus. Depending upon where the nucleus was hit, the angle from which it was deflected differed. (5)

1. An atom was much more than just empty space of scattered electrons. (as opposed to what the "plum pudding model" proposed)
2. An atom must have a positively charged center that contains most of its matter (5). 
                  - He called this dense, concentrated center the nucleus (4). 
3. The positively charged center (nucleus) was relatively small in reference to the total size of the atom (5). 

Importance/ Significance:
            Rutherford’s gold foil experiment proved the “plum pudding model” of the atom incorrect, which allowed for a more complete understanding of the atom. His discovery of the nucleus and atomic structure was refined by Niels Bohr. Niels Bohr designed a model of the atom based off of Rutherford’s experiment, which is often referred to as the Rutherford Bohr model. This is so important because this is the basic atomic model that is still being used toady (3). The discovery of the nucleus and atomic model also allowed for the development of nuclear physics (4). This highly contributed to discoveries of the atomic and nuclear bomb, as well as organizing the Manhattan Project (2). In conclusion, Rutherford’s gold foil experiment contributed to what is today’s atomic model, and there has yet to be another discovery to disprove it.

 (1) “Ernest Rutherford – Biography”. 5 Oct 2010 - bio.html
(2)  “Ernest Rutherford, first baron (1871 – 1937)”. October 5, 2010
(3) Pestka, Jessica. “About Rutherford’s Gold Foil Experiment”. October 5, 2010
(4) “An Overview of Rutherford’s Gold Foil Experiment”. October 5, 2010
(5)“The Gold Foil Experiment”. October 5, 2010