Rutherford Gold Foil Expirement - Geiger-Marsden Experiment

May 4, 2015

Geiger-Marsden Experiment

        From between the years 1908 and 1913, scientists Hans Geiger and Ernest Marsden changed the way the world looked at the atom. Through careful experimentation under Ernest Rutherford they were able to prove that every atom contains a positively charged nucleus that contains most of the atoms mass. This experiment is known as the Geiger-Marsden experiment or the Gold Foil experiment and is one of the most important experiments in the field of chemistry and atomic structure.

        Prior to Ernest Rutherford’s experiment, little was known about the structure of the atom. For most of history it was believed that the building blocks of matter were made out of homogenous particles. This was believed due to an idea of Greek philosopher Democritus dating back to 460 B.C. He theorized that when matter is broken down there must be a point at which it can no longer be broken down further and that the furthest it can be broken down to are small particles of matter. (Atomic Theory) Until the 1800’s, not much research had been done to disprove that theory. In the early 1800’s scientist John Dalton performed many experiments with different elements from which he derived his own atomic theory. This theory consisted of the idea that all elements are made out of small fundamental particles called atoms and that all atoms of the same element are the same in size and mass and atoms of different elements differ in size and mass. Additionally, he theorized that atoms could not be subdivided, created or destroyed which was later proven to be wrong. (Clemens)

Thought it was known that atoms exist, little was know about their makeup. In 1904, following the discovery of the negatively charged electron, physicist J.J. Thomson proposed a model of the atom called the “Plum pudding model”. (Discovery of Electrons) Within this model, Thomson proposed that since an atom has a neutral charge, something must balance out the negative charge of the electrons. (Fouad) He therefore theorized that the atom consisted of a positively charged soup or pudding like substance in which electrons are dispersed throughout. Only a few years later, Ernest Rutherford disproved J.J. Thomson’s theory with the Gold Foil Experiment. (Coupenhagen)

Ernest Rutherford was a professor at the University of Manchester in the early 1900’s. Ernest Marsden was an undergraduate student studying under Rutherford at that time. Hans Geiger was a fellow physicist who joined Rutherford in 1906. Beginning with an experiment in 1908, Rutherford sought out to measure the charge-to-mass ratio of alpha particles with the help of his colleagues. Alpha particles are positively charged particles that are emitted by some substances. Rutherford used radium as his source for alpha particles but; before determining the charge-to-mass ratio, he first needed to know how many alpha particles radium gave off. To figure this out he and his team devised a system that would create and count an electrical pulse created by focusing the alpha particles into a glass tube containing two electrodes. The results however proved unreliable, as the air molecules within the tube would interfere with the particles and results would be inconsistent. Changes to the original experiment would be performed in the years to come. Additionally, the particles scattered in peculiar ways. This led Rutherford to question how much matter was needed to scatter the alpha rays. (Fouad)

In 1910, Rutherford devised a new experiment that would yield better and more reliable results and hopefully show how the alpha particles were scattered. The experiment entailed bombarding a thin sheet of foil with the alpha rays. A florescent screen would then be used to map out the trajectory of the particles by determining where on the screen the particles hit. After testing many types of metals, gold was chosen as the final test element. This is because gold is extremely malleable and can be hammered out to nearly 0.0004 centimeters thick: that is nearly one atom in thickness. (Wilbraham) This would yield better results, as the particles would have to travel through less to get to their target. Geiger and Marsden did the bulk of the work and collected most of the results by request of Rutherford. What was collected was astonishing. As the alpha particles made contact with the gold foil, most went directly through, however many seemed to bounce off in all different directions. Some of the particles even bouncing directly back at the source. This was entirely unexpected. According to Thomson’s plum pudding model, the alpha particles should have passed directly through the foil with paths only deviating slightly. The evidence that was collected seemed to disprove Thomson’s theory of the atom. (Wilbraham)