Geiger-Marsden experiment

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Geiger-Marsden experiment

The Geiger–Marsden experiment (also called the Gold foil experiment or the Rutherford experiment) was an experiment to probe the structure of the atom

Atom

|-! bgcolor=gray | Properties|-||}The atom is a basic unit of matter consisting of a dense, central atomic nucleus surrounded by a electron cloud of electric charge electrons....
performed by Hans Geiger

Hans Geiger

Johannes Wilhelm Geiger was a Germany physicist. He is perhaps best known as the co-inventor of the Geiger counter and for the Geiger-Marsden experiment which discovered the atomic nucleus....
and Ernest Marsden

Ernest Marsden

Sir Ernest Marsden was a England-New Zealand physicist. He was born in Lancashire and educated at Queen Elizabeth's Grammar School, Blackburn, where an inter-house trophy rewarding academic excellence bears his name....
in 1909, under the direction of Ernest Rutherford

Ernest Rutherford

Ernest Rutherford, 1st Baron Rutherford of Nelson, Order of Merit , Royal Society was a New Zealand-born British chemist who became known as the father of nuclear physics....
at the Physical Laboratories of the University of Manchester

University of Manchester

The University of Manchester is a "red brick university" civic university located in Manchester, England. It is a member of the Russell Group of large research-intensive universities and the N8 Group for research collaboration....
. The unexpected results of the experiment demonstrated for the first time the existence of the atomic nucleus

Atomic nucleus

Plum pudding model

The plum pudding model of the atom by J.J. Thomson, who discovered the electron in 1897, was proposed in 1904 before the discovery of the atomic nucleus....
of the atom, and the development of the Rutherford (or planetary) model

Rutherford model

The Rutherford model or planetary model is a model of the atom devised by Ernest Rutherford. Rutherford directed the famous Geiger-Marsden experiment in , which suggested to Rutherford's analysis that the Plum pudding model of the atom was incorrect....
.

am of alpha particle

Alpha particle

Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium atomic nucleus; hence, it can be written as He2+ or 42He2+....
s, generated by the radioactive decay

Radioactive decay

Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide....
of radium

Radium

Radium is a radioactive chemical element which has the symbol Ra and atomic number 88. Its appearance is almost pure white, but it readily oxidizes on exposure to air, turning black....
, was directed normally onto a sheet of very thin gold

Gold

Gold is a chemical element with the symbol Au and atomic number 79. It is a highly sought-after precious metal, having been used as money, as a store of value, in jewelry, in sculpture, and for ornamentation since the beginning of recorded history....
foil.

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Experimental procedure and results

A beam of alpha particle

Alpha particle

Radioactive decay

Radium

Gold

Zinc sulfide

Zinc sulfide is a chemical compound with the chemical formula ZincSulfur. Zinc sulfide is a white- to yellow-colored powder or crystal. It is typically encountered in the more stable cubic form, known also as zinc blende or sphalerite....
(ZnS) which was used as a detector: the ZnS sheet would light up when hit with alpha particles. Under the prevailing plum pudding model, the alpha particles should all have been deflected by, at most, a few degrees; measuring the pattern of scattered particles was expected to provide information about the distribution of charge

Electric charge

Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields....
within the atom. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees. According to Rutherford:

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. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge.

Conclusions

Rutherford interpreted the experimental results in a famous 1911 paper. He was able to definitively reject J.J. Thomson's plum pudding model of the atom, since none of Thomson's negative "corpuscles" (i.e. electron

Electron

The electron is a subatomic particle that carries a negative electric charge. It has elementary particle and is believed to be a point particle....
s) contained enough charge or mass to deflect alphas strongly, nor did the diffuse positive "pudding" or cloudlike positive charge, in which the electrons were embedded in the plum pudding model. Instead, Rutherford suggested that a large amount of the atom's charge and mass is instead concentrated into a very physically-small (as compared with the size of the atom) region, giving it a very high electric field. Outside of this "central charge" (later termed the nucleus

Atomic nucleus

The nucleus of an atom is the very dense region, consisting of nucleons , at the center of an atom. Although the size of the nucleus varies considerably according to the mass of the atom, the size of the entire atom is comparatively constant....
), he proposed that the atom was mostly empty space. Rutherford was unable to say from the experiment whether the nuclear charge was positive or negative, but used the following language for pictorial purposes:

"For concreteness, consider the passage of a high speed a particle through an atom having a positive central charge Ne, and surrounded by a compensating charge of N electrons."

From energetic considerations of how far alpha particles of known speed would be able to penetrate toward a central charge of 100 e, Rutherford was able to calculate that the radius of his gold central charge would need to be less (how much less could not be told) than 3.4 x 10^-14 metres (the modern value is only about a fifth of this). This was in a gold atom known to be 10^-10 metres or so in radius – a very surprising finding, as it implied a strong central charge less than 1/3000th of the diameter of the atom.

Although Rutherford's model of the atom had a number of problems which were only resolved following the development of quantum mechanics

Quantum mechanics

Quantum mechanics is a set of principles underlying the most fundamental known description of all physical systems at the microscopic scale . Notable amongst these principles are both a dual wave-like and particle-like behavior of matter and radiation, and prediction of probabilities in situations where classical physics predicts certaintie...
, the central conclusion from the Geiger–Marsden experiment, the existence of the nucleus, still holds.

Geiger-Marsden experiment

Experimental procedure and results

Conclusions

See also

External links