rutherford goldfolien experiment

What is the 'Gold Foil Experiment'? The Geiger-Marsden experiments explained

Physicists got their first look at the structure of the atomic nucleus.

J.J. Thomson model of the atom

Gold foil experiments, rutherford model of the atom.

• The real atomic model

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Bibliography.

The Geiger-Marsden experiment, also called the gold foil experiment or the α-particle scattering experiments, refers to a series of early-20th-century experiments that gave physicists their first view of the structure of the atomic nucleus and the physics underlying the everyday world. It was first proposed by Nobel Prize -winning physicist Ernest Rutherford.

As familiar as terms like electron, proton and neutron are to us now, in the early 1900s, scientists had very little concept of the fundamental particles that made up atoms . 

In fact, until 1897, scientists believed that atoms had no internal structure and believed that they were an indivisible unit of matter. Even the label "atom" gives this impression, given that it's derived from the Greek word "atomos," meaning "indivisible." 

But that year, University of Cambridge physicist Joseph John Thomson discovered the electron and disproved the concept of the atom being unsplittable, according to Britannica . Thomson found that metals emitted negatively charged particles when illuminated with high-frequency light. 

His discovery of electrons also suggested that there were more elements to atomic structure. That's because matter is usually electrically neutral; so if atoms contain negatively charged particles, they must also contain a source of equivalent positive charge to balance out the negative charge.

By 1904, Thomson had suggested a "plum pudding model" of the atom in which an atom comprises a number of negatively charged electrons in a sphere of uniform positive charge,  distributed like blueberries in a muffin. 

The model had serious shortcomings, however — primarily the mysterious nature of this positively charged sphere. One scientist who was skeptical of this model of atoms was Rutherford, who won the Nobel Prize in chemistry for his 1899 discovery of a form of radioactive decay via α-particles — two protons and two neutrons bound together and identical to a helium -4 nucleus, even if the researchers of the time didn't know this.

Rutherford's Nobel-winning discovery of α particles formed the basis of the gold foil experiment, which cast doubt on the plum pudding model. His experiment would probe atomic structure with high-velocity α-particles emitted by a radioactive source. He initially handed off his investigation to two of his protégés, Ernest Marsden and Hans Geiger, according to Britannica . 

Rutherford reasoned that if Thomson's plum pudding model was correct, then when an α-particle hit a thin foil of gold, the particle should pass through with only the tiniest of deflections. This is because α-particles are 7,000 times more massive than the electrons that presumably made up the interior of the atom.

Marsden and Geiger conducted the experiments primarily at the Physical Laboratories of the University of Manchester in the U.K. between 1908 and 1913. 

The duo used a radioactive source of α-particles facing a thin sheet of gold or platinum surrounded by fluorescent screens that glowed when struck by the deflected particles, thus allowing the scientists to measure the angle of deflection. 

The research team calculated that if Thomson's model was correct, the maximum deflection should occur when the α-particle grazed an atom it encountered and thus experienced the maximum transverse electrostatic force. Even in this case, the plum pudding model predicted a maximum deflection angle of just 0.06 degrees. 

Of course, an α-particle passing through an extremely thin gold foil would still encounter about 1,000 atoms, and thus its deflections would be essentially random. Even with this random scattering, the maximum angle of refraction if Thomson's model was correct would be just over half a degree. The chance of an α-particle being reflected back was just 1 in 10^1,000 (1 followed by a thousand zeroes). 

Yet, when Geiger and Marsden conducted their eponymous experiment, they found that in about 2% of cases, the α-particle underwent large deflections. Even more shocking, around 1 in 10,000 α-particles were reflected directly back from the gold foil.

Rutherford explained just how extraordinary this result was, likening it to firing a 15-inch (38 centimeters) shell (projectile) at a sheet of tissue paper and having it bounce back at you, according to Britannica  

Extraordinary though they were, the results of the Geiger-Marsden experiments did not immediately cause a sensation in the physics community. Initially, the data were unnoticed or even ignored, according to the book "Quantum Physics: An Introduction" by J. Manners.

The results did have a profound effect on Rutherford, however, who in 1910 set about determining a model of atomic structure that would supersede Thomson's plum pudding model, Manners wrote in his book.

The Rutherford model of the atom, put forward in 1911, proposed a nucleus, where the majority of the particle's mass was concentrated, according to Britannica . Surrounding this tiny central core were electrons, and the distance at which they orbited determined the size of the atom. The model suggested that most of the atom was empty space.

When the α-particle approaches within 10^-13 meters of the compact nucleus of Rutherford's atomic model, it experiences a repulsive force around a million times more powerful than it would experience in the plum pudding model. This explains the large-angle scatterings seen in the Geiger-Marsden experiments.

Later Geiger-Marsden experiments were also instrumental; the 1913 tests helped determine the upper limits of the size of an atomic nucleus. These experiments revealed that the angle of scattering of the α-particle was proportional to the square of the charge of the atomic nucleus, or Z, according to the book "Quantum Physics of Matter," published in 2000 and edited by Alan Durrant.  

In 1920, James Chadwick used a similar experimental setup to determine the Z value for a number of metals. The British physicist went on to discover the neutron in 1932, delineating it as a separate particle from the proton, the American Physical Society said . 

What did the Rutherford model get right and wrong?

Yet the Rutherford model shared a critical problem with the earlier plum pudding model of the atom: The orbiting electrons in both models should be continuously emitting electromagnetic energy, which would cause them to lose energy and eventually spiral into the nucleus. In fact, the electrons in Rutherford's model should have lasted less than 10^-5 seconds. 

Another problem presented by Rutherford's model is that it doesn't account for the sizes of atoms. 

Despite these failings, the Rutherford model derived from the Geiger-Marsden experiments would become the inspiration for Niels Bohr 's atomic model of hydrogen , for which he won a Nobel Prize in Physics .

Bohr united Rutherford's atomic model with the quantum theories of Max Planck to determine that electrons in an atom can only take discrete energy values, thereby explaining why they remain stable around a nucleus unless emitting or absorbing a photon, or light particle.

Thus, the work of Rutherford, Geiger  (who later became famous for his invention of a radiation detector)  and Marsden helped to form the foundations of both quantum mechanics and particle physics. 

Rutherford's idea of firing a beam at a target was adapted to particle accelerators during the 20th century. Perhaps the ultimate example of this type of experiment is the Large Hadron Collider near Geneva, which accelerates beams of particles to near light speed and slams them together. 

• See a modern reconstruction of the Geiger-Marsden gold foil experiment conducted by BackstageScience and explained by particle physicist Bruce Kennedy . 
• Find out more about the Bohr model of the atom which would eventually replace the Rutherford atomic model. 
• Rutherford's protege Hans Gieger would eventually become famous for the invention of a radioactive detector, the Gieger counter. SciShow explains how they work .

Thomson's Atomic Model , Lumens Chemistry for Non-Majors,.

Rutherford Model, Britannica, https://www.britannica.com/science/Rutherford-model

Alpha particle, U.S NRC, https://www.nrc.gov/reading-rm/basic-ref/glossary/alpha-particle.html

Manners. J., et al, 'Quantum Physics: An Introduction,' Open University, 2008. 

Durrant, A., et al, 'Quantum Physics of Matter,' Open University, 2008

Ernest Rutherford, Britannica , https://www.britannica.com/biography/Ernest-Rutherford

Niels Bohr, The Nobel Prize, https://www.nobelprize.org/prizes/physics/1922/bohr/facts/

House. J. E., 'Origins of Quantum Theory,' Fundamentals of Quantum Mechanics (Third Edition) , 2018

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Robert Lea is a science journalist in the U.K. who specializes in science, space, physics, astronomy, astrophysics, cosmology, quantum mechanics and technology. Rob's articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University

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About Rutherford's Gold Foil Experiment

Five Types of Atomic Models

Ernest Rutherford, originally from New Zealand, is credited as being the father of nuclear physics for his discoveries in atomic structure, even though Hantaro Nagaoka, a physicist from the Imperial University of Tokyo, first proposed the theory of the nucleus as it is known today. Rutherford's "gold foil experiment" led to the discovery that most of an atom's mass is located in a dense region now called the nucleus. Prior to the groundbreaking gold foil experiment, Rutherford was granted the Nobel Prize for other key contributions in the field of chemistry.

The popular theory of atomic structure at the time of Rutherford's experiment was the "plum pudding model." This model was developed in 1904 by J.J. Thompson, the scientist who discovered the electron. This theory held that the negatively charged electrons in an atom were floating in a sea of positive charge--the electrons being akin to plums in a bowl of pudding. Although Dr. Nagaoka had published his competing theory that electrons orbit a positive nucleus, akin to the way the planet Saturn is orbited by its rings, in 1904, the plum pudding model was the prevailing theory on the structure of the atom until it was disproved by Ernest Rutherford in 1911.

The gold foil experiment was conducted under the supervision of Rutherford at the University of Manchester in 1909 by scientist Hans Geiger (whose work eventually led to the development of the Geiger counter) and undergraduate student Ernest Marsden. Rutherford, chair of the Manchester physics department at the time of the experiment, is given primary credit for the experiment, as the theories that resulted are primarily his work. Rutherford's gold foil experiment is also sometimes referred to as the Geiger-Marsden experiment.

The gold foil experiment consisted of a series of tests in which a positively charged helium particle was shot at a very thin layer of gold foil. The expected result was that the positive particles would be moved just a few degrees from their path as they passed through the sea of positive charge proposed in the plum pudding model. The result, however, was that the positive particles were repelled off of the gold foil by nearly 180 degrees in a very small region of the atom, while most of the remaining particles were not deflected at all but rather passed right through the atom.

Significance

The data generated from the gold foil experiment demonstrated that the plum pudding model of the atom was incorrect. The way in which the positive particles bounced off the thin foil indicated that the majority of the mass of an atom was concentrated in one small region. Because the majority of the positive particles continued on their original path unmoved, Rutherford correctly deducted that most of the remainder of the atom was empty space. Rutherford termed his discovery "the central charge," a region later named the nucleus.

Rutherford's discovery of the nucleus and proposed atomic structure was later refined by physicist Niels Bohr in 1913. Bohr's model of the atom, also referred to as the Rutherford Bohr model, is the basic atomic model used today. Rutherford's description of the atom set the foundation for all future atomic models and the development of nuclear physics.

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Who did the Gold Foil Experiment?

The gold foil experiment was a pathbreaking work conducted by scientists Hans Geiger and Ernest Marsden under the supervision of Nobel laureate physicist Ernest Rutherford that led to the discovery of the proper structure of an atom . Known as the Geiger-Marsden experiment, it was performed at the Physical Laboratories of the University of Manchester between 1908 and 1913.

The prevalent atomic theory at the time of the research was the plum pudding model that was developed by Lord Kelvin and further improved by J.J. Thomson. According to the theory, an atom was a positively charged sphere with the electrons embedded in it like plums in a Christmas pudding.

With neutrons and protons yet to be discovered, the theory was derived following the classical Newtonian Physics. However, in the absence of experimental proof, this approach lacked proper acceptance by the scientific community.

What is the Gold Foil Experiment?

Description.

The method used by scientists included the following experimental steps and procedure. They bombarded a thin gold foil of thickness approximately 8.6 x 10 -6 cm with a beam of alpha particles in a vacuum. Alpha particles are positively charged particles with a mass of about four times that of a hydrogen atom and are found in radioactive natural substances. They used gold since it is highly malleable, producing sheets that can be only a few atoms thick, thereby ensuring smooth passage of the alpha particles. A circular screen coated with zinc sulfide surrounded the foil. Since the positively charged alpha particles possess mass and move very fast, it was hypothesized that they would penetrate the thin gold foil and land themselves on the screen, producing fluorescence in the part they struck.

Like the plum pudding model, since the positive charge of atoms was evenly distributed and too small as compared to that of the alpha particles, the deflection of the particulate matter was predicted to be less than a small fraction of a degree.

Observation

Though most of the alpha particles behaved as expected, there was a noticeable fraction of particles that got scattered by angles greater than 90 degrees. There were about 1 in every 2000 particles that got scattered by a full 180 degree, i.e., they retraced their path after hitting the gold foil.

Simulation of Rutherford’s Gold Foil Experiment Courtesy: University of Colorado Boulder

The unexpected outcome could have only one explanation – a highly concentrated positive charge at the center of an atom that caused an electrostatic repulsion of the particles strong enough to bounce them back to their source. The particles that got deflected by huge angles passed close to the said concentrated mass. Most of the particles moved undeviated as there was no obstruction to their path, proving that the majority of an atom is empty.

In addition to the above, Rutherford concluded that since the central core could deflect the dense alpha particles, it shows that almost the entire mass of the atom is concentrated there. Rutherford named it the “nucleus” after experimenting with various gases. He also used materials other than gold for the foil, though the gold foil version gained the most popularity.

He further went on to reject the plum pudding model and developed a new atomic structure called the planetary model. In this model, a vastly empty atom holds a tiny nucleus at the center surrounded by a cloud of electrons. As a result of his gold foil experiment, Rutherford’s atomic theory holds good even today.

Rutherford’s Atomic Model

Rutherford’s Gold Foil Experiment Animation

• Rutherford demonstrated his experiment on bombarding thin gold foil with alpha particles contributed immensely to the atomic theory by proposing his nuclear atomic model.
• The nuclear model of the atom consists of a small and dense positively charged interior surrounded by a cloud of electrons.
• The significance and purpose of the gold foil experiment are still prevalent today. The discovery of the nucleus paved the way for further research, unraveling a list of unknown fundamental particles.
• Chemed.chem.purdue.edu
• Chem.libretexts.org
• Large.stanford.edu
• Radioa ctivity.eu.com

Article was last reviewed on Friday, February 3, 2023

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5 responses to “Gold Foil Experiment”

Super very much helpful to me,clear explanation about every act done by our Rutherford that is under different sub headings ,which is very much clear to ,to study .very much thanks to the science facts.com.thank u so much.

Good explanation,very helpful ,thank u ,so much

very clear and helpful, perfect for my science project!

Thank you for sharing the interactive program on the effects of the type of atom on the experiment! Looking forward to sharing this with my ninth graders!

Rutherford spearheaded with a team of scientist in his experiment of gold foil to capture the particles of the year 1911. It’s the beginning of explaining particles that float and are compacted . Rutherford discovered this atom through countless experiments which was the revolutionary discovery of the atomic nuclear . Rutherford name the atom as a positive charge and the the center is the nucleus.

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Discovering the Nucleus: Rutherford’s Gold Foil Experiment

History of Chemistry: Rutherford Gold Foil Experiment

In this article, you will learn the history behind the Rutherford Gold Foil Experiment and the events that led to the discovery of the atomic nucleus. If you enjoy this article, check out our other history of chemistry articles linked below!

• Rutherford Atomic Model
• JJ Thompson cathode-ray tube
• Rutherfords Jar Experiment
• Molecular Geometry tutorial
• The structure of an atom
• Bohr Atomic Model
• Nuclear Reactions

Who was Ernest Rutherford?

Ernest Rutherford is known as the father of nuclear physics. Born in Brightwater, New Zealand on August 30th, 1871, Rutherford was the fourth of twelve children. His father was a farmer and his mother a school teacher. From a very early age, Rutherford understood the importance of hard work and the power of education. In school, he excelled greatly and at the age of fifteen won an academic scholarship to study at Nelson Collegiate School. Then, at the age of 19, he won another academic scholarship to study at Canterbury College in Christchurch. A few years later he won another scholarship, the exhibition science scholarship, and he left New Zealand to study at Trinity College, Cambridge in England. While there, he conducted research at the Cavendish Laboratory under his advisor J.J. Thomson .

During his time at Cavendish Lab, Rutherford faced adversity from his peers. Because he was from New Zealand, he was often ostracized by fellow students. In the end, he used this as motivation to succeed. Which he did as he made a multitude of great discoveries through his research in gases and radioactivity. These included the discovery of different types of radiation, radiometric dating, and the nucleus of an atom.

The Rutherford Gold Foil Experiment

The experiment.

While working as a chair at the University of Manchester, Rutherford conducted the gold-foil experiment alongside Hans Geiger and Ernest Marsden. In this experiment, they shot alpha particles –which Rutherford had discovered years prior– directly at a piece of thin gold foil . As the alpha particles passed through, they would hit the phosphorescent screen encasing the foil. When the particles came into contact with the screen, there would be a flash.

Observations

Going into the experiment, Rutherford had formed preconceptions for the experiment based on J.J. Thomson’s plum pudding model . He predicted the alpha particles would shoot through the foil with ease. Some of the particles did manage to pass directly through the foil, but some veered from the path either bouncing back or deflecting. Rutherford found this to be an exciting observation and compared it to shooting a bullet at a piece of tissue and having it bounce back.

From this observation, two deductions were made. Firstly, he concluded most of the atom is composed of empty space. Secondly, he concluded there must be something small, dense, and positive inside the atom to repel the positively charged alpha particles. This became the nucleus, which in Latin means the seed inside of a fruit.

The Nuclear Model

The gold-foil experiment disproved J.J. Thomsons plum pudding model, which hypothesized the atom was positively charged spaced with electrons embedded inside. Therefore, giving way to the nuclear model. In this model, Rutherford theorized the atomic structure was similar to that of the solar system. Where the nucleus was in this middle and surrounded by empty space with orbiting electrons.

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What is the Rutherford gold-foil experiment?

A piece of gold foil was hit with alpha particles , which have a positive charge. Most alpha particles went right through. This showed that the gold atoms were mostly empty space. Some particles had their paths bent at large angles. A few even bounced backward. The only way this would happen was if the atom had a small, heavy region of positive charge inside it.

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• Structure of Atom
• Size Of The Nucleus

Rutherford's Experiment - Size of the Nucleus

Introduction

Understanding the fundamental structure of matter is essential to Physics. Figuring out the size of the nucleus, which is the crux of this article, would not be possible without the Rutherford gold foil experiment. The Rutherford model of the atom was the first correct interpretation of the atom, and it laid the groundwork for Bohr to build his interpretation.

Rutherford Gold Foil Experiment

Before Rutherford’s experiment, the best model of the atom that was known to us was the Thomson or “plum pudding” model. In this model, the atom was believed to consist of a positive material “pudding” with negative “plums” distributed throughout. Later, Rutherford’s alpha-particle scattering experiment changed our perception of the atomic structure. Rutherford directed beams of alpha particles at thin gold foil to test this model and noted how the alpha particles scattered from the foil.

In the experiment, Rutherford showed us that the atom was mainly empty space with the nucleus at the centre and electrons revolving around it. When  alpha particles  were fired towards the gold foil, Rutherford noticed that 1 in 20000 particles underwent a change in direction of motion of more than 90 degrees. The rest 19999 particles deviated from their trajectory by a very small margin. This led to the conclusion that the atom consisted of an empty space with most of the mass concentrated at the centre in tiny volumes. This volume at the centre was named ‘the nucleus’; Latin for ‘little nut’.

Through this experiment, Rutherford made 3 observations as follows:

• Highly charged alpha particles went straight through the foil undeflected. This would have been the expected result for all of the particles if the plum pudding model was correct.
• Some alpha particles were deflected back through large angles.
• A very small number of alpha particles were deflected backwards! To this, Rutherford remarked, “It was as incredible as if you fired a 15-inch shell at a piece of tissue paper, and it came back at you!”

To explain these observations, a new model of the atom was needed. In the new model, the positive material was considered to be concentrated in a small but massive region called the nucleus. Electrons were considered to be revolving around the nucleus, preventing one atom from trespassing on its neighbour’s space to complete this model.

Size of the Nucleus

It was possible to obtain the size of the nucleus through Rutherford’s experiment. We can calculate the size of the nucleus, by obtaining the point of closest approach of an alpha particle. By shooting alpha particles of kinetic energy 5.5 MeV, the point of closest approach was estimated to be about 4×10 -14 m. Since the repulsive force acting here is Coulomb repulsion, there is no contact. This means that the size of the nucleus is smaller than 4×10 -14 m.

The sizes of the nuclei of various elements have been accurately measured after conducting many more iterations of the experiment. Having done this, a formula to measure the size of the nucleus was determined.

\(\begin{array}{l}R = R_0 A^{\frac{1}{3}}\end{array} \)

Where R 0 = 1.2×10 -15 m.

From the formula, we can conclude that the volume of the nucleus, which is proportional to R 3 , is proportional to A (mass number). Another thing to be noticed in the equation is that there is no mention of density in the equation. This is because the density of the nuclei does not vary with elements.  The density of the nucleus is approximately 2.3×10 17 kg.m -3 .

Top 15 Most Important and Expected Questions on Nuclei in Hindi.

Frequently Asked Questions – FAQs

What did rutherford’s gold foil experiment teach us about the atomic structure.

Rutherford’s gold foil experiment showed us that the atom is mostly empty space with a comparatively tiny, massive, positively charged nucleus in the centre.

What caused the alpha particles to deflect in Rutherford’s gold-foil experiment?

Rutherford thought that the particles would fly straight through the foil. However, he found that the particle’s path would be shifted or deflected when passing through the foil. This is because like charges repel each other.

How did Rutherford’s experiment affect our world?

Rutherford’s experiment gave us a better, more practical understanding of matter. The experiment provided conclusive evidence against previous conceptions of matter and provided a new model consistent with the facts.

Why gold foil is used in Rutherford experiment?

Gold foil is used because of its elevated malleability in Rutherford’s ray scattering experiment. The very thin gold foil is used in the experiment, and gold can be shaped into very thin films.

How did Rutherford’s experiment work?

The Rutherford Gold Foil experiment fired at a thin layer of gold with minute particles. A small proportion of the particles have been observed to have been deflected, while a remainder has gone through the layer. This led Rutherford to infer that at its core, the mass of an atom was concentrated.

You might want to read the following articles

• Bohr’s Model Of An Atom
• Rutherford Atomic Model
• Thomson’s Atomic Model

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How you calculate here closest apporoach by just knowing the speed of alpha partical

The distance of closest approach is defined as the distance of the charged particle from the center of the nucleus at which the whole of the initial kinetic energy of charged particle gets converted into electric potential energy of the system. When an alpha particle with a kinetic energy E is fired at a gold nucleus it will feel a repulsion which increases as it gets closer. When all the kinetic energy has been converted to potential energy the alpha particle (charge q) has reached its distance of closest approach (d 0 ) and comes to rest.

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What did Rutherford's gold-foil experiment tell about the atom?

In order for the alpha particles to be deflected, they would have to hit or come near to a positively charged particle in the atom. These experiments led Rutherford to describe the atom as containing mostly empty space, with a very small, dense, positively charged nucleus at the center, which contained most of the mass of the atom, with the electrons orbiting the nucleus.

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Impact of this question

RUTHERFORD-Experiment

Das Wichtigste auf einen Blick

• Im RUTHERFORDschen Streuversuch wird eine dünne Metallfolie mit \(\alpha\)-Teilchen (positiv geladen) beschossen.
• Auf Basis des THOMSONschen Atommodells wird erwartet, dass alle \(\alpha\)-Teilchen die dünne Metallfolie unabgelenkt passieren.
• Entgegen den Erwartungen werden einige wenige \(\alpha\)-Teilchen von der Folie sogar zurückgestreut.
• Die Ergebnisse führen zum RUTHERFORDschen Atommodell.

Ernest RUTHERFORD untersuchte seit 1898 Alpha-Teilchen. Er war mit dieser neuartigen Strahlung völlig vertraut und konnte später auch nachweisen, dass es sich dabei um Helium-Kerne handelte. 1909 wurde er mit einer ziemlich unverständlichen Eigenschaft der Alphateilchen konfrontiert, auf die seine Schüler Hans GEIGER und Ernest MARSDEN bei Streuversuchen gestoßen waren.

GEIGER und MARSDEN beschossen 1909 eine sehr dünne Goldfolie mit einem eng begrenzten Strahl von Alphateilchen über mehrere Monate (sie führten diese Untersuchungen bis 1913 weiter). GEIGER verwendete eine Dicke von 8,6·10 -6 cm für die Folie. Tatsächlich war die Folie so dünn, dass sie auf ein Gitter aufgebracht wurde (bei dem Gitter ohne Folie wurden keine Reflexionen von Alphateilchen festgestellt, es war für Alphastrahlung durchsichtig).

Aufbau und Durchführung

Ein radioaktives Präparat P befindet sich in einer Bleiummantelung, es sendet Alphateilchen aus, die auf eine sehr dünne Metallfolie (mehrere 1000 Atomlagen dick) treffen.

Die durchgehenden bzw. gestreuten Alphateilchen werden mit Hilfe eines Szintillationsschirms nachgewiesen. Dieser leuchtet kurzzeitig an der Stelle auf, wo er von einem Alphateilchen getroffen wird.

Zur genauen Lokalisation dient das Mikroskop, welches hinter dem Szintillationsschirm angeordnet ist. Das Mikroskop samt Szintillationsschirm kann um die Goldfolie gedreht werden.

Die Zählrate ΔN der in einer bestimmten Zeiteinheit auf den Schirm treffenden Alphateilchen wird in Abhängigkeit vom Drehwinkel \(\vartheta \) (Ablenkwinkel der Alphateilchen) bestimmt.

Abb. 2 zeigt vorne den Druckdeckel, seitlich hinten die Vakuumanschlüsse und der Einschusskanal für schnelle Teilchen aus einem Beschleuniger und oben die Registriergeräte (drehbar)

Erwartete Beobachtung

Zum Zeitpunkt der Durchführung des Experiments von GEIGER und MARSDEN war bereits bekannt, dass das Atom einen Radius von etwa \(10^{-10}\,\rm{m}\) hat, sich in ihm negativ geladene Elektronen befinden und es nach außen elektrisch neutral ist und somit auch eine positive Ladung beinhalten muss. Beim damals vorherrschenden THOMSONschen Modell (auch Wassermelonenmodell oder Rosinenkuchenmodell genannt) sollte sich die ganze Atommasse homogen auf den Raum verteilen, den das Atom in Anspruch nahm.

Nach diesem Modell müssten alle Alphateilchen ungehindert durch die Atome der Goldfolie dringen und alle auf der gegenüberliegenden Seite des Alpha-Strahlers ankommen. Dieses Verhalten stellt die Animation in Abb. 4 schematisch dar.

Tatsächlich gemachte Beobachtungen

Die Formulierungen der Beobachtungen sind angelehnt an die Originalarbeit von 1911.

1. Nahezu alle Alphateilchen gingen durch die Goldfolie hindurch, so als wäre sie nicht da. Diese Alphateilchen bewegten sich geradlinig weiter, bis sie auf die Wand oder den Detektor aufschlugen.

2. Einige wenige Alphateilchen wurden geringfügig abgelenkt, üblicherweise um einen Winkel von 2° und weniger. GEIGER fand heraus, dass ein Alphateilchen im Durchschnitt an einem Goldatom um \(0{,}005^\circ \) abgelenkt wird. Die wahrscheinlichste Ablenkung an der ganzen Goldfolie lag unter einem Grad (RUTHERFORD benannte sie in seiner Veröffentlichung von 1911 mit \(0{,}87^\circ \)).

3. Ganz wenige Teilchen wurden um einen Winkel von mehr als \(90^\circ \) abgelenkt. (1 von 8000 bei einer Platinfolie, RUTHERFORD nannte in seiner Veröffentlichung von 1911, dass es 1 von 20 000 bei der verwendeten Goldfolie sind). Dieses Ergebnis veranlasste RUTHERFORD zu der oft zitierten Aussage:

"Es war bestimmt das unglaublichste Ergebnis, das mir je in meinem Leben widerfuhr. Es war fast so unglaublich, als wenn einer eine 15-Zoll-Granate auf ein Stück Seidenpapier abgefeuert hätte und diese zurückgekommen wäre und ihn getroffen hätte."

Teilchenzahl unter verschiedenen Streuwinkeln

Das Diagramm in Abb. 6 zeigt den Zusammenhang zwischen dem Ablenkwinkel \(\vartheta \) und der Anzahl \(\Delta N\) der in einen bestimmten Raum- und Zeitbereich gestreuten Alphateilchen. Beachte, dass es sich bei der y-Achse um keinen linearen Maßstab handelt.

Das Diagramm zeigt, dass der größte Teil der auf die Folie treffenden Alphateilchen die Folie nahezu unabgelenkt passieren konnte. Es kommen aber sehr selten auch Streuungen mit Streuwinkeln größer als 90° vor.

Hinweis: Die Erklärung der Großwinkelstreuung als Folge mehrerer hintereinander erfolgten Kleinwinkelstreuungen an verschiedenen Goldkernen ist nicht haltbar, da es aufgrund der sehr dünnen Folie unwahrscheinlich ist, dass ein Alphateilchen beim Durchdringen der Folie sehr nah an mehreren Kernen vorbeifliegt.

Erklärung von Rutherford

Die Beobachtung der stark abgelenkten Alphateilchen konnte mit dem Atommodell von THOMSON nicht erklärt werden.

RUTHERFORDs Lösung des Problems der Erklärung von der Streuung der Alphateilchen an Goldatomen sowohl um kleine als auch um große Winkel lag im Atomkern. RUTHERFORD erklärte, dass die ganze Atommasse im Zentrum des Atoms auf einem sehr kleinen Raum vereinigt sei. Dies nannte er den Atomkern.

In seiner Veröffentlichung von 1911 schlug er vor, den Kern als einen Punkt zu betrachten: "Wir müssen uns die Masse und die positive Ladung sowohl des Atoms als auch des Alphateilchens in einem Punkt konzentriert vorstellen, dessen Ausmessungen weniger als \(10^{-14}\,\rm{m}\) sind." RUTHERFORD verwendete in seiner Veröffentlichung nicht das Wort "Kern". Er nannte es "Ladungskonzentration". In einem Artikel von 1912 schrieb er einige Seiten über das Atommodell und verwendete dort erstmals das Wort "Kern".

• Am Anfang seiner Überlegungen legte sich Rutherford zunächst noch gar nicht über das Vorzeichen der Ladung des Kerns fest. Er sprach nur von einer " zentralen Ladung ".
• Später legte er sich auf eine positive Zentralladung fest: ". . . an atom having a positive central charge N ·e, and surrounded by a compensating charge of N electrons . . . "

Erläuterung des Experimentes mit dem Rutherfordschen Atommodell

Die Animation in Abb. 8 zeigt die moderne (Atomkern aus positiv geladenen Protonen und elektrisch neutralen Neutronen) Erklärung des Streuexperiments von GEIGER und MARSDEN: Diejenigen Alphateilchen, die weit genug von Kernen entfernt die Folie durchdringen, werden so gut wie nicht abgelenkt. Nur die wenigen Geschosse, welche sehr nahe an einem Kern vorbeifliegen, erfahren eine nennenswerte Ablenkung. Dabei gilt: Je kleiner der Abstand zum Kern, desto größer der Streuwinkel.

Insgesamt erklärt das Atommodell von RUTHERFORD mit dem Atomkern die drei wesentlichen Ergebnisse des Experiments von GEIGER und MARSDEN:

1. Der Kern ist so klein, dass sich die überwältigende Mehrheit aller Alphateilchen ohne jegliche Ablenkung durch die Goldfolie hindurchbewegt, als wäre dort nichts. Es sieht so aus als wäre das Atom ein hauptsächlich leerer Raum. Die Wechselwirkung der geladenen Alphateilchen mit den Hüllenelektronen führt wegen der relativ hohen kinetischen Energie der Alphateilchen und der im Vergleich zu den Elektronen viel größeren Masse zu keiner nennenswerten Ablenkung.

2. Nur einige Alphateilchen gelangen beim Durchgang durch die Folie so nahe an einen Atomkern, dass die elektrische Abstoßung der beiden positiv geladenen Körperchen zu einer geringfügigen Ablenkung des Alphateilchens um ein oder zwei Grad führt. Die Wahrscheinlichkeit für diese Nähe zum Kern und die daraus resultierende Ablenkung wurden durch RUTHERFORD und seine Mitarbeiter berechnet und mit den Versuchsergebnissen verglichen. Es gab eine sehr gute Übereinstimmung.

3. Nur ganz wenige Alphateilchen treffen fast direkt auf die Mitte des Atoms. Die Alphateilchen, die sich mit etwa 10 % der Lichtgeschwindigkeit dem Atomkern nähern, werden durch dessen elektrische Kräfte vollelastisch "reflektiert". Da das Alphateilchen wesentlich leichter als der Goldkern ist, wird es auf einer hyperbelförmigen Bahn "reflektiert", so dass es um einen Winkel von 90° und mehr abgelenkt wird. Der Kern nimmt bei diesem Stoß nur geringfügig Energie von den Alphateilchen auf, die aber nicht ausreicht, um das Atom aus dem Metallverbund zu lösen.

Erläuterungen im Video

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Rutherford Streuversuch

Möchtest du mehr über den Aufbau und die Bedeutung vom Rutherford Streuversuch  erfahren? Dann schau dir dazu unser Video und diesen Artikel an!

Rutherford Streuversuch einfach erklärt 

Rutherford streuversuch aufbau , streuversuch erwartungen, streuversuch beobachtungen, streuversuch schlussfolgerungen, rutherford atommodell, atommodelle.

Beim Rutherford Streuversuch werden Alphateilchen auf eine Goldfolie gestrahlt. Die Alphateilchen sind positiv geladene Heliumkerne. Sie haben keine Elektronen. Ziel des Versuchs ist es zu sehen, ob und wie die Alphateilchen abgelenkt werden. Um die Bewegung der Teilchen nach dem Auftreffen auf die Goldfolie zu sehen, gibt es einen Leuchtschirm um die Folie herum.

Nach der damaligen Vorstellung vom Atomaufbau, war die Erwartung des Versuchs, dass die Teilchen einfach durch die Folie durchgelassen werden. Es wurden aber Teilchen abgelenkt und sogar zurückgestreut.

Deshalb hat Ernest Rutherford ein neues Atommodell eingeführt. Dabei ist der größte Unterschied beim Rutherford Atommodell , dass es im Inneren des Atoms einen sehr kleinen, positiv geladenen Atomkern beinhaltet.

Der Aufbau des Rutherford Streuversuch besteht im wesentlich aus einem Strahl aus Alphateilchen , einer Goldfolie und einem Leuchtschirm .

Die Quelle der Alphastrahlung ist Radium. Radium ist radioaktiv und setzt deswegen durch radioaktiven Zerfall positiv geladene Alphateilchen frei. Alphateilchen sind zweifach positive Heliumkerne mit keinem Elektron.

Aus einer Öffnung in einem Bleibehälter strahlen die Alphateilchen auf eine dünne Goldfolie. Dort können die Teilchen entweder abgelenkt werden oder dringen durch die Goldfolie durch. Um zu erkennen, wie sich die Teilchen verhalten, ist ein kreisförmiger Leuchtschirm um die Folie platziert. Treffen die Teilchen auf den Schirm, leuchten sie nämlich mit einem kleinen Lichtblitz auf.

Aufgrund des damaligen Thomsonschen Atommodells hatte Rutherford die Erwartung, dass die meisten Alphateilchen in einer geraden Linie durch die Goldfolie durchdringen.

Beim Modell von Thomson ging man nämlich davon aus, dass das Atom elektrisch neutral ist und seine positive Ladung wie auch die Masse gleichmäßig auf das Atom verteilt ist. Die Elektronen liegen an festen Plätzen innerhalb des Atoms. Das kannst du dir wie Rosinen im Kuchen vorstellen (Rosinenkuchenmodell) .

Wenn die gesamte Masse des Atoms also gleichmäßig verteilt ist, sollten die Alphateilchen dank ihrer Größe und Energie ungehindert durch das Atom dringen. Wegen der gleichmäßigen Verteilung der Ladungen ist das Atom insgesamt neutral, weshalb die Alphateilchen auch durch die Ladung nicht abgelenkt werden sollten. 

Anders als erwartet sind nicht alle Alphateilchen ungehindert durch die Goldfolie gedrungen. Die meisten Teilchen konnten in gerader Linie hinter der Goldfolie auf dem Schirm beobachtet werden. Einige Teilchen wurden aber auch etwas nach links und rechts abgelenkt . Entgegen der Erwartungen wurden sehr wenige Teilchen sogar zurückgestreut !

Beim Rutherford Streuversuch konnte auch beobachtet werden, dass die Ablenkung mit hohen Ablenkwinkeln deutlich seltener war. Daher wurden auch nur wenige Alphateilchen genau entgegen der Einstrahlrichtung zurückgestreut.

Die Beobachtungen des Rutherfordschen Streuversuchs stimmten nicht mit den Erwartungen überein. Weil die Teilchen sehr selten zurückgestreut wurden, muss der größte Teil der Masse in einem sehr kleinen Punkt gesammelt sein, zum Beispiel in einem Atomkern . Die Chance, dass ein Teilchen direkt auf den Kern trifft, ist also eher gering.

Wenn die gesamte Masse im Atomkern ist, wäre dort auch die gesamte positive Ladung. Dementsprechend erklärt Rutherford die Ablenkung der positiven Teilchen damit, dass sie sehr nah am Atomkern vorbeigeflogen sind und durch die positive Ladung abgestoßen, also abgelenkt wurden.

D er Rest des Atoms ist laut Rutherford dann mit den Elektronen gefüllt und ansonsten „leer“ . Der Atomkern ist also nur von der Hülle des Atoms und den Elektronen umgeben. Durch diese „Leere“ in der Elektronenhülle und weil die Elektronen so viel kleiner als Alphateilchen sind, konnte der Großteil der Teilchen ohne jegliche Ablenkung hinter der Goldfolie ankommen.

All diese Erkenntnisse widersprachen  dem Thomsonschen Modell, weshalb Rutherford ein eigenes Atommodell entwarf. Das Rutherford Atommodell.

Beim Rutherford Atommodell  sind alle Schlussfolgerungen aus dem Rutherfordscher Streuversuch mit eingeflossen. Demnach hat ein Atom laut Rutherford:

• einen sehr kleinen, positiv geladenen Atomkern, der fast die gesamte Masse des Atoms enthält
• eine fast 3000-mal größere Atomhülle, welche zum größten Teil „leer“ ist und aus „Nichts“ besteht
• Elektronen, die um den Atomkern kreisen und die positive Ladung vom Atomkern nach außen abschirmen

Dadurch ist das Atommodell Rutherfords viel näher am heutigen Verständnis des Atoms als Thomsons Modell.

Probleme des Rutherford Atommodells

Obwohl du mit dem Rutherfordschen Atommodell  vieles zum Atomaufbau erklären kannst, stößt es schnell an seine Grenzen . So umfasst es zum Beispiel nicht die unterschiedlichen Energieniveaus der Elektronen. Doch sind sie nötig, um die Spektrallinien verschiedener Gase zu erklären.

Zudem ist es auch nicht möglich, mit dem Rutherfordschen Atommodell zu begründen, warum die Elektronen nicht in den Kern stürzen , wenn sie um den Atomkern kreisen. Elektronen auf einer Kreisbahn sind beschleunigte Ladungen und geben deshalb Energie in Form von Strahlung ab. Laut dem Atommodell Rutherfords ist von Strahlung aber keine Rede. Demnach würden die Elektronen nicht in einer stabilen Kreisbahn um den Atomkern kreisen. Stattdessen würden sie nach und nach in den Atomkern stürzen.

Durch die vielen Probleme des Rutherfordschen Atommodells wurde es später durch das Bohrsche Atommodell und anschließend das Schalenmodell abgelöst. Willst du noch mehr über die vielen verschiedenen Atommodelle erfahren? Welche es gibt, erklären wir dir in einem Video ! Schau doch gleich rein! 

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