Top: Expected results: alpha particles passing through the plum pudding model of the atom undisturbed.
Bottom: Observed results: a small portion of the particles were deflected, indicating a small, concentrated positive charge.

The Gold foil experiment, or Geiger-Marsden experiment was an experiment done by Hans Geiger and Ernest Marsden in 1909, under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester which led to the downfall of the plum pudding model of the atom. Image File history File links Download high resolution version (508x785, 146 KB) Summary Top: Expected results of Rutherfords gold foil experiment: alpha particles passing through the plum pudding model of the atom undisturbed. ... Image File history File links Download high resolution version (508x785, 146 KB) Summary Top: Expected results of Rutherfords gold foil experiment: alpha particles passing through the plum pudding model of the atom undisturbed. ... Johannes ( Hans ) Wilhelm Geiger (September 30, 1882 – September 24, 1945) was a German physicist. ... Sir Ernest Marsden (1888 - 1970), was a British-New Zealand physicist. ... 1909 (MCMIX) was a common year starting on Friday (see link for calendar). ... Ernest Rutherford, 1st Baron Rutherford of Nelson, OM, FRS (August 30, 1871 - October 19, 1937), called father of nuclear physics, pioneered the orbital theory of the atom notably in his discovery of rutherford scattering off the nucleus with his gold foil experiment. ... The University of Manchester in Manchester, England is a university that was formed from the merger of the Victoria University of Manchester (commonly known as the University of Manchester before the merger) and UMIST (University of Manchester Institute of Science and Technology) on 1 October 2004. ... A schematic representation of the plum pudding model of the atom. ...

They measured the deflection of alpha particles directed normally onto a sheet of very thin gold foil. Under the prevailing plum pudding model, the alpha particles should all have been deflected by at most of a few degrees. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees. From this Rutherford concluded that the atom contained a very small positive charge which could repel the alpha particles if they came close enough, subsequently developed into the Bohr model. An alpha particle is deflected by a magnetic field Alpha particles or alpha rays (named after the first letter in the greek alphabet) are a form of particle radiation which are highly ionizing and have low penetration. ... The Bohr model of the atom In atomic physics, the Bohr model depicts the atom as a small, positively charged nucleus surrounded by electrons in orbit - similar in structure to the solar system. ...

Methodology

Geiger and Marsden bombarded a number of different metal foils with alpha particles generated from a tube of radium bromide gas. A low power microscope was used to count the scattering of these particles, a procedure requiring many hours in a darkened room watching for tiny flashes of light as the scattered particles struck a zinc sulfide scintillant screen. An alpha particle is deflected by a magnetic field Alpha particles or alpha rays (named after the first letter in the greek alphabet) are a form of particle radiation which are highly ionizing and have low penetration. ... 1852 microscope Compound microscope made by John Cuff in 1750 A microscope (Greek: micron = small and scopos = aim) is an instrument for viewing objects that are too small to be seen by the naked or unaided eye. ... Zinc sulfide is a chemical compound with the formula ZnS. Zinc sulfide is a white to yellow colored powder or crystal. ... The term Scintillation has several different meanings, including: Atmospheric induced Scintillation effects which influence astronomical observations. ...

A variety of different foils were used such as aluminium, iron, gold and lead along with different thicknesses of gold foil made by packing several pieces of very thin foil together. Given the very high mass and momentum of an alpha particle, the expectation was that the particles would pass through having being deflected by a tiny angle at most, with the number of particles penetrating falling off as the thickness of foil (and the atomic weight of its material) was increased; the remainder being absorbed. General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13, 3, p Appearance silvery Atomic mass 26. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... General Name, Symbol, Number gold, Au, 79 Chemical series transition metals Group, Period, Block 11, 6, d Appearance metallic yellow Atomic mass 196. ... General Name, Symbol, Number lead, Pb, 82 Chemical series poor metals Group, Period, Block 14, 6, p Appearance bluish white Atomic mass 207. ... Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. ... In physics, momentum is a physical quantity related to the velocity and mass of an object. ...

However they were astonished to find that although this was generally true, around 1 in 8000 particles were reflected through more than 90 degrees even with a single sheet of extremely thin, 6x10^-8 metre (or about 200 atoms) thick, gold foil, an observation completely at odds with the predictions of the plum pudding model.

Conclusions

The result was completely unpredicted, prompting Rutherford to later comment "It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you".

Early in 1911 Rutherford published a revised model of the atom, known as the Rutherford atom. The observations indicated that a model of the atom with a diffuse positive charge was incorrect and that it was instead concentrated. He concluded that the atom is mostly empty space, with most of the atom's mass concentrated in a tiny centre, the nucleus and electrons being held in orbit around it by electrostatic attraction. The nucleus was around 10^-15 metres in diameter, in the centre of a 10^-10 metre diameter atom. Those alpha particles that had come into close proximity with the nucleus had been strongly deflected whereas the majority had passed at a relatively great distance to it. 1911 was a common year starting on Sunday (click on link for calendar). ... A stylized representation of a lithium atom. ...

Rutherford's model was developed by Niels Bohr into the Bohr model proposed in 1913. The Rutherford atom had a number of problems, in particular electrons should radiate electromagnetic energy and rapidly spiral into the nucleus. Niels Bohr Niels Henrik David Bohr (October 7, 1885 – November 18, 1962) was a Danish physicist who made essential contributions to understanding atomic structure and quantum mechanics. ... The Bohr model of the atom In atomic physics, the Bohr model depicts the atom as a small, positively charged nucleus surrounded by electrons in orbit - similar in structure to the solar system. ... 1913 (MCMXIII) is a common year starting on Wednesday. ...

The experiment failed to yield any data concerning electrons (which had to be present in order to make the atom electrically neutral). Rutherford did surmise that they would have a very small mass, and that they would be randomly distributed around the nucleus.

See also

• List of famous experiments

The following is a list of historically important scientific experiments and observations. ...

References

• On a Diffuse Reflection of the α-Particles, H. Geiger and E. Marsden, Proceedings of the Royal Society, 1909 A vol. 82, p. 495-500
• The Scattering of Alpha and Beta Particles by Matter and the Structure of the Atom, E. Rutherford, Philosophical Magazine, 6th Series, 21:669 (1911)
• Description of the experiment, from the New Mexico Institute of Mining and Technology
• Short biography of Ernest Rutherford