Rutherford’s Gold Foil Experiment

Rutherford’s Apparatus Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry. beam of alpha particles radioactive substance MODERN ALCHEMY “Ernest Rutherford (1871-1937) was the first person to bombard atoms artificially to produce transmutated elements. The physicist from New Zealand described atoms as having a central nucleus with electrons revolving around it. He showed that radium atoms emitted “rays” and were transformed into radon atoms. Nuclear reactions like this can be regarded as transmutations – one element changing into another, the process alchemists sought in vain to achieve by chemical means.” Eyewitness Science “Chemistry” , Dr. Ann Newmark, DK Publishing, Inc., 1993, pg 35 When Rutherford shot alpha particles at a thin piece of gold foil, he found that while most of them traveled straight through, some of them were deflected by huge angles. circular ZnS - coated fluorescent screen gold foil Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

Rutherford ‘Scattering’ He fired a (alpha) particles at a very thin sample of gold foil According to the Thomson model the a particles would only be slightly deflected Rutherford discovered that they were deflected through large angles and could even be reflected straight back to the source particle source Lead collimator Gold foil a q Rutherford’s results strongly suggested that both the mass and positive charge are concentrated in a tiny fraction of the volume of the atom, called the nucleus. Rutherford established that the nucleus of the hydrogen atom was a positively charged particle, which he called a proton. Also suggested that the nuclei of elements other than hydrogen must contain electrically neutral particles with the same mass as the proton. The neutron was discovered in 1932 by Rutherford’s student Chadwick. Because of Rutherford’s work, it became clear that an α particle contains two protons and neutrons—the nucleus of a helium atom.

Rutherford’s Apparatus beam of alpha particles radioactive substance MODERN ALCHEMY Ernest Rutherford (1871-1937) was the first person to bombard atoms artificially to produce transmutated elements. The physicist from New Zealand described atoms as having a central nucleus with electrons revolving around it. He showed that radium atoms emitted “rays” and were transformed into radon atoms. Nuclear reactions like this can be regarded as transmutations – one element changing into another, the process alchemists sought in vain to achieve by chemical means. Eyewitness Science “Chemistry” , Dr. Ann Newmark, DK Publishing, Inc., 1993, pg 35 Ernest Rutherford English physicist. (1910) Wanted to see how big atoms are. Used radioactivity, alpha particles - positively charged pieces given off by polonium atoms. Shot them at a thin gold foil (~0.5 um thick) which can be made a few atoms thick. When the alpha particles hit a florescent screen, it glows. Approximately 1/20,000 bounced back at the alpha emitter source. Rutherford said this was like shooting a 15" shell at tissue paper and the shell came back and hit you. It was clearly, NOT what he thought should happen if Thomson's model of the atom was correct. Ernest Rutherford received the 1908 Nobel prize in chemistry for his work at McGill University with radioactive substances. fluorescent screen circular - ZnS coated gold foil Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

What He Expected The alpha particles to pass through without changing direction (very much) Because the positive charges were spread out evenly. Alone they were not enough to stop the alpha particles California WEB

What he expected… California WEB

Because, he thought the mass was evenly distributed in the atom -

What he got… richocheting alpha particles

The Predicted Result: expected path expected marks on screen Observed Result: mark on screen likely alpha particle path

Interpreting the Observed Deflections . gold foil . beam of alpha particles undeflected particles . . The observations: (1) Most of the alpha particles pass through the foil un-deflected. (2) Some alpha particles are deflected slightly as the penetrate the foil. (3) A few (about 1 in 20,000) are greatly deflected. (4) A similar small number do not penetrate the foil at all, but are reflected back toward the source. Rutherford believed that when positively charged alpha particles passed near the positively charged nucleus, the resulting strong repulsion caused them to be deflected at extreme angles. Rutherford's interpretation: If atoms of the foil have a massive, positively charged nucleus and light electrons outside the nucleus, one can explain how: (1) an alpha particle passes through the atom un-deflected (a fate share by most of the alpha particles); (2) an alpha particle is deflected slightly as it passes near an electron; (3) an alpha particle is strongly deflected by passing close to the atomic nucleus; and (4) an alpha particle bounces back as it approaches the nucleus head-on. deflected particle Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

Density and the Atom Since most of the particles went through, the atom was mostly empty. Because the alpha rays were deflected so much, the positive pieces it was striking were heavy. Small volume and big mass = big density This small dense positive area is the nucleus California WEB

Explanation of Alpha-Scattering Results + - Alpha particles Nuclear atom Nucleus Plum-pudding atom Thomson’s model Rutherford’s model

Summary of Results Discovered the nucleus of the atom. Positive dense core of the atom (later found to be composed of protons and neutrons) The atom is mostly empty space where the electrons orbit the nucleus. The nuclear model https://www.youtube.com/watch?v=XBqHkraf8iE