Ernest Rutherford Student of J.J. Thomson (1894) Early studies based on radioactivity Discovered that some materials emit radiation  Alpha particle: Eventually was confirmed to be helium nuclei, which meant an alpha particle was simply 2 protons and 2 neutrons  Like Thomson, Rutherford was interested in the deflection patterns of alpha particles when exposed to electric & magnetic fields  Found the deflection patterns by measuring alpha particles position on photographic film  By accident, he noticed that if the alpha particles passed through a thin sheet of mica, the images on the film were blurred

Rutherford and alpha particles The images were sharp if the mica was not present  Something about the mica sheet was causing the alpha particles to scatter at seemingly random small angles, resulting in blurred images

Rutherford & Geiger Wanted to study the effects of alpha particles with matter  Had to develop a way to count individual alpha particles when they hit the screen  Found that a screen coated with zinc sulfide emitted a flash of light each time it was hit by an alpha particle  Rutherford & Geiger had to sit in a dark room for hours and individually count the flashes of light Rutherford asked Geiger to measure the angles of deflection when the alpha particles were passed through a thin ( cm) sheet of gold foil  When the gold sheet was bombarded with alpha particles, Geiger found that the scattering was small  Results were consistent with Rutherford’s expectations. Because he knew that alpha particles had a considerable mass and moved quite rapidly, he anticipated that virtually all of the alpha particles would go through the metal foil without much disruption

Rutherford, Geiger, & Marsden Marsden was a graduate student working in Rutherford’s lab and Geiger had suggested that a research project should be given to Marsden  Rutherford: “why not let him see whether any alpha particles can be scattered through a large angle?”  Marsden had found that a small fraction (perhaps 1 in 20,000) of the alpha particles were scattered through angles larger than 90 degrees Rutherford was in awe  “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!”

What could the results from this gold foil experiment mean? The + charge in an atom of gold was known, but if this charge were collected on a sphere the size of an atom (like Thomson suggested), the repulsion would be far too weak  To explain the force experienced by the alpha particle, the charge and much of the mass would have to be collected in a much smaller sphere  Published results in 1911 and proposed a model for the atom that is still accepted today  All of the positive charge and essentially of the mass of the atom is concentrated in an incredibly small fraction of the total volume of the atom, which he called the nucleus (Latin for “little nut”)

Further findings from Rutherford’s experiment Most of the alpha particles were able to pass right through A small fraction came close to the nucleus of a gold atom as they passed through and were slightly deflected from the positive-positive repulsion of the alpha particle and nucleus But occasionally, an alpha particle would run directly into the nucleus and would result in a great repulsion that deflected the alpha particle through an angle of 90 degrees or more  By carefully measuring the fraction of the alpha particles deflected through large angles, Rutherford was able to estimate the size of the nucleus (JUST LIKE WE DID!)  Found that the radius of the nucleus is at least 10,000 times smaller than the radius of the atom  The vast majority of the atom is therefore empty space!!

Someone throw out the plum pudding!! Rutherford revised Thomson’s plum pudding model, showing how electrons could orbit a positively charged nucleus, like planets orbiting a sun Because the majority of the “plum-pudding” atom would be electrically neutral (no charge), the alpha particles would have no problem shooting through Rutherford’s atom Nuclear Model

Modern version of Rutherford's experiment