The Rutherford’s Experiment and the Geiger Marsden experiment were both the same experiment, where Rutherford being the mentor of Geiger and Marsden in the investigation of the experiment at the University of Manchester in 1909. The experiment was so important because it changes our view and understanding upon the structure of the atom. Before Rutherford, there was a man named Joseph John Thomson who proposed the structure of the atom. He stated that the electrons of the atoms are actually inside the atom, and the atom itself is positively charged, like a “raisin pudding”. The electrons are scattered all around the atom. He stated that the electrons and the positively charged material cancel each other, making it stable. The structure of the atom, proposed by Thomson. (J.J Thomson Plum Pudding Model, Batul Nafisa Baxamusa, 2011. Retrieved on June 26th, 2012 from URL: http://www.buzzle.com/articles/labeled-atom-diagram.html)
9 important things that would get 9 marks:
-Why’s are answered (Why gold foil was used, etc)
-How’s are answered (how do we fire the alpha particle, etc) -The aim of the experiment
-Why was the experiment important to us?
-Is the new structure of atom correct?
The aim of the experiment was to elucidate the structure of the atom, as there were so many theories about the structure of the atom at that time, which was championed by J.J Thomson. Rutherford and his team wanted to find out how the alpha particle would react against the thin foil. Geiger and Marsden constructed a lead box (to prevent radiation spreading all over the vacuum) with a hole on the center of the box, and inserted an alpha particle emitter (decaying polonium-214) inside the box. About five centimeters in front of the hole on the lead box was a gold foil with approximately 8.6x10-6 thick. The apparatus was then surrounded by zinc sulfide screen, which would glow when the alpha particle hit it. The experiment was then done in a vacuum in complete darkness, to ensure that the observer could see the glowing alpha particle. They used gold leaf foil because gold leaf foil is highly malleable, thus can be cut to micro sizes. It was the best material for the job, as alpha particles only penetrates through thin materials, and such small size would ensure that all the alpha particles make it though the gold leaf foil. Geiger and Marsden then would fire the alpha particle upon the gold foil. Most of the alpha particle went straight across the gold foil, consistent with J.J Thomson’s theory, because the positive charge of the alpha particle and the negative charge of the gold foil would attract each other, making the alpha particle passes through easily. Some of the alpha particles actually bounces a little bit off track, because the electrons of the alpha particle would occasionally meet with the electrons of the gold foil. They repel each other, but because of the speed of the alpha particle, the inclination of the alpha particle was not big and therefore not significant. But, surprisingly, few of the alpha particles actually bounce off the predicted track of where it should’ve gone. The alpha particles sometimes create a ninety-degree bounce, and some even bounces right back to the shooter! J.J Thomson could not explain this phenomenon using his model of atom, as such deflection would require a concentrated energy of the positive charged material, while the positively charged material was big and therefore it was not concentrated enough to hold such energy to repel another very fast travelling positively charged material.
A picture describing Rutherford’s experiment (Rutherford’s Experiment and Atomic Model, David Darling, 2011. Taken on July 23rd, 2012 from URL: http://www.daviddarling.info/index.html#latestnews) Rutherford then made a conclusion that the positively charged material (proton) was concentrated on the center of the atom, with the electrons surrounding it from a far distance. This implies that the atom consisted of mostly empty space. So, even though the alpha particle was moving very fast, it can still be deflected with high inclination or even bounces back, because the protons are concentrated, thus they would have high energy to deflect each other on such high degrees. Now, our understanding upon the atom’s structure has gotten a further step better, even though the Rutherford’s structure of atom is not absolutely correct. With this model, the electrons would emit energy to repel against the proton (positive and negative charges attract each other). The energy would eventually be gone and the electron finally collides with the nucleus. From this result, it can be concluded that Rutherford’s model of atom couldn’t be the conclusive model of the atom.
1. Rutherford’s experiment, Oracle Thinkquest, 2009. Taken on July 23rd, 2012 from URL: http://library.thinkquest.org/19662/low/eng/exp-rutherford.html 2. The Rutherford’s experiment, 2011. Taken on July 12th, 2012 from URL: http://www.physics.rutgers.edu/meis/Rutherford.htm 3. The Rutherford’s experiment, Michael W. Davidson and the Florida State University, 1995-2012. Taken on June 25th, 2012 from URL: http://micro.magnet.fsu.edu/electromag/java/rutherford/ 4. Rutherford’s Experiment and Atomic Model, David Darling, 2008. Taken on July 22nd, 2012 from URL: http://www.daviddarling.info/encyclopedia/R/Rutherfords_experiment_and_atomic_model.html