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What was Shown by Rutherford’s Gold Foil Experiment
Posted By Jim J Jones On October 7, 2013 @ 6:40 am In Chemistry
Ernest Rutherford’s 1911 experiments with gold foil discovered that the atom’s positive charge is concentrated
in one place – a nucleus, as we now know it. Although Rutherford did not personally conduct the experiment
(his assistants, Ernest Marsden and Hans Geiger, of “Geiger Counter” fame, actually carried it out), he did
design and direct it. The groundbreaking results ensured that the “Rutherford model”, as it was then known,
gained quick, widespread acceptance in the scientific community.
Rutherford’s goal had not been to revolutionize chemistry. He was one of the greatest experimentalists of his
time, but this experiment was merely intended to study the distribution of charge in the atom. The logic was that
if an alpha particle with a charge of 2+ is fired at a super-thin gold foil (on the order of micrometers), the degree
to which it is thrown off course allows you to reverse-engineer the structure of the atom that it passed through.
Rutherford was in for a big surprise when he found that the vast majority of his alpha particles went undisturbed,
but that a select few came zooming straight back at the alpha emitter. He would later describe this as being like
firing a cannon at tissue paper, and having the cannon ball fly back in your face. His reverse engineering told
him that there was only one possible explanation: the atom’s positive charge was all at one location. And so the
nucleus was discovered.
Rutherford’s results flew in the face of contemporary science. The “plum pudding” model of the atom, which
dictated that protons and electrons are spread out evenly throughout the atom (like fruit n a plum pudding), was
in complete contradiction to Rutherford’s findings. It was not long before the new way prevailed, and
Rutherford had the brand-new “nuclear model” of the atom named after himself. Of course, the great Danish
theorist Niels Bohr would unseat him barely a decade later – but that’s another story.
Rutherford’s model treats the atom as a solar system, of sorts. It argued (incorrectly, as Bohr and Louis de
Broglie would later show) that the nucleus acted as a sun, located at the center. Instead of using gravity to attract
the electrons, it relied on the electric force. The electrons orbited the nucleus, zipping around at speeds where
the inward pull of the electric force equaled the outward pull of the centripetal force.
Even to this day, some of the fundamental ideas of Rutherford’s model are still used. Modern knowledge of the
nucleus is not all that much better than it was at Rutherford’s time, though neutrons have been added to the
modern model. Also, the idea that the electrons move in “orbitals” is, to some limited extent, still relevant, and
is an especially useful mental tool for those who cannot visualize
URL to article: http://www.sciences360.com/index.php/what-was-shown-by-rutherfords-gold-foilexperiment-2-12404/
Who?
Name the scientists involved, and
how they were related.
What?
Explain the results of the
experiment.
When & Where?
When and where was the
experiment conducted?
Why?
Why was the experiment conducted?
How?
How did the results of the
experiment change existing Atomic
Theory?
What Did Rutherford's Gold Foil Experiment Tell Scientists About Atomic Structure?
By Eric Moll, eHow Contributor
Rutherford's experiment, also known as the gold foil experiment or the Geiger-Marsden experiment, involved firing positively charged alpha particles, which are basically helium nuclei, at a very thin sheet
of gold foil surrounded by a detector. The experiment, devised by Ernest Rutherford and performed by Hans Geiger and Ernest Marsden in 1911, yielded surprising results which disproved the prevailing
model of atomic structure, generally known as the plum pudding model.
1. Experimental Setup
o The experiment setup of the gold foil experiment consisted of an alpha particle emitter, a sheet of super-thin gold foil, and a circular zinc-sulfide detector sheet which surrounded the
gold foil. The alpha particles were produced by the radioactive decay of radium. The zinc-sulfide sheet would light up when struck by an alpha particle. Because the initial angle of the
alpha particles was controlled by a narrow slit in front of the emitter, the angle of deflection could be calculated based on where they hit the detector.
Expected Results
o The plum pudding model of atomic structure claimed that an atom consisted of negatively charged electrons dispersed relatively evenly throughout a soup or cloud of diffuse positive
charge. If this model were true, the alpha particles would pass through the gold foil without much hindrance. There would be no electrical forces strong enough to change their path by
more than a few degrees.
Actual Results
o Rather than seeing only minor changes to the paths of the alpha particles, the detector sheet showed that most of the particles went straight through without any interference but some
were deflected at large angles or even deflected backwards. This was surprising because under the diffuse plum pudding model, there wouldn't be any localized electrical charges strong
enough to deflect a heavy, fast-moving alpha particle.
Rutherford Model
o The results of the gold foil experiment allowed Rutherford to develop a new theory of atomic structure in which nearly all of the mass and all of the positive charge of an atom is
contained in a very small "nucleus" at the center of the atom, roughly 1/4000 the diameter of the whole atom. The negative charges, or electrons, orbit around this nucleus.
Most of the atom is empty space. The Rutherford Model of atomic structure has since been revised due to advances in quantum mechanics but its basic premise of a tiny, central,
positively charged nucleus remains unchanged to this day.
Rutherford’s gold foil experiment
Purpose:
Expected results: (explain why)
Actual Results: (explain why)
What did the results of this experiment add to the existing atomic theory?