gold foil experiment based on

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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Experimental Evidence for the Structure of the Atom

George sivulka march 23, 2017, submitted as coursework for ph241 , stanford university, winter 2017, introduction.

The Rutherford Gold Foil Experiment offered the first experimental evidence that led to the discovery of the nucleus of the atom as a small, dense, and positively charged atomic core. Also known as the Geiger-Marsden Experiments, the discovery actually involved a series of experiments performed by Hans Geiger and Ernest Marsden under Ernest Rutherford. With Geiger and Marsden's experimental evidence, Rutherford deduced a model of the atom, discovering the atomic nucleus. His "Rutherford Model", outlining a tiny positively charged atomic center surrounded by orbiting electrons, was a pivotal scientific discovery revealing the structure of the atoms that comprise all the matter in the universe.

The experimental evidence behind the discovery involved the scattering of a particle beam after passing through a thin gold foil obstruction. The particles used for the experiment - alpha particles - are positive, dense, and can be emitted by a radioactive source. Ernest Rutherford discovered the alpha particle as a positive radioactive emission in 1899, and deduced its charge and mass properties in 1913 by analyzing the charge it induced in the air around it. [1] As these alpha particles have a significant positive charge, any significant potential interference would have to be caused by a large concentration of electrostatic force somewhere in the structure of the atom. [2]

Previous Model of the Atom

The scattering of an alpha particle beam should have been impossible according to the accepted model of the atom at the time. This model, outlined by Lord Kelvin and expanded upon by J. J. Thompson following his discovery of the electron, held that atoms were comprised of a sphere of positive electric charge dotted by the presence of negatively charged electrons. [3] Describing an atomic model similar to "plum pudding," it was assumed that electrons were distributed throughout this positive charge field, like plums distributed in the dessert. However, this plum pudding model lacked the presence of any significant concentration of electromagnetic force that could tangibly affect any alpha particles passing through atoms. As such, alpha particles should show no signs of scattering when passing through thin matter. [4] (see Fig. 2)

The Geiger Marsden Experiments

Testing this accepted theory, Hans Geiger and Ernest Marsden discovered that atoms indeed scattered alpha particles, a experimental result completely contrary to Thompson's model of the atom. In 1908, the first paper of the series of experiments was published, outlining the apparatus used to determine this scattering and the scattering results at small angles. Geiger constructed a two meter long glass tube, capped off on one end by radium source of alpha particles and on the other end by a phosphorescent screen that emitted light when hit by a particle. (see Fig. 3) Alpha particles traveled down the length of the tube, through a slit in the middle and hit the screen detector, producing scintillations of light that marked their point of incidence. Geiger noted that "in a good vacuum, hardly and scintillations were observed outside of the geometric image of the slit, "while when the slit was covered by gold leaf, the area of the observed scintillations was much broader and "the difference in distribution could be noted with the naked eye." [5]

On Rutherford's request, Geiger and Marsden continued to test for scattering at larger angles and under different experimental parameters, collecting the data that enabled Rutherford to further his own conclusions about the nature of the nucleus. By 1909, Geiger and Marsden showed the reflection of alpha particles at angles greater than 90 degrees by angling the alpha particle source towards a foil sheet reflector that then would theoretically reflect incident particles at the detection screen. Separating the particle source and the detector screen by a lead barrier to reduce stray emission, they noted that 1 in every 8000 alpha particles indeed reflected at the obtuse angles required by the reflection of metal sheet and onto the screen on the other side. [6] Moreover, in 1910, Geiger improved the design of his first vacuum tube experiment, making it easier to measure deflection distance, vary foil types and thicknesses, and adjust the alpha particle stream' velocity with mica and aluminum obstructions. Here he discovered that both thicker foil and foils made of elements of increased atomic weight resulted in an increased most probable scattering angle. Additionally, he confirmed that the probability for an angle of reflection greater than 90 degrees was "vanishingly small" and noted that increased particle velocity decreased the most probably scattering angle. [7]

Rutherford's Atom

Backed by this experimental evidence, Rutherford outlined his model of the atom's structure, reasoning that as atoms clearly scattered incident alpha particles, the structure contained a much larger electrostatic force than earlier anticipated; as large angle scattering was a rare occurrence, the electrostatic charge source was only contained within a fraction of the total volume of the atom. As he concludes this reasoning with the "simplest explanation" in his 1911 paper, the "atom contains a central charge distributed through a very small volume" and "the large single deflexions are due to the central charge as a whole." In fact, he mathematically modeled the scattering patterns predicted by this model with this small central "nucleus" to be a point charge. Geiger and Marsden later experimentally verified each of the relationships predicted in Rutherford's mathematical model with techniques and scattering apparatuses that improved upon their prior work, confirming Rutherford's atomic structure. [4, 8, 9] (see Fig. 1)

With the experimentally analyzed nature of deflection of alpha rays by thin gold foil, the truth outlining the structure of the atom falls into place. Though later slightly corrected by Quantum Mechanics effects, the understanding of the structure of the the atom today almost entirely follows form Rutherford's conclusions on the Geiger and Marsden experiments. This landmark discovery fundamentally furthered all fields of science, forever changing mankind's understanding of the world around us.

© George Sivulka. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

[1] E. Rutherford, "Uranium Radiation and the Electrical Conduction Produced By It," Philos. Mag. 47 , 109 (1899).

[2] E. Rutherford, "The Structure of the Atom," Philos. Mag. 27 , 488 (1914).

[3] J. J. Thomson, "On the Structure of the Atom: an Investigation of the Stability and Periods of Oscillation of a Number of Corpuscles Arranged at Equal Intervals Around the Circumference of a Circle; with Application of the Results to the Theory of Atomic Structure," Philos. Mag. 7 , 237 (1904).

[4] E. Rutherford, "The Scattering of α and β Particles by Matter and the Structure of the Atom," Philos. Mag. 21 , 669 (1911).

[5] H. Geiger, "On the Scattering of the α Particles by Matter," Proc. R. Soc. A 81 , 174 (1908).

[6] H. Geiger and E. Marsden, "On a Diffuse Reflection of the α-Particles," Proc. R. Soc. A 82 , 495 (1909).

[7] H. Geiger, "The Scattering of the α Particles by Matter," Proc. R. Soc. A 83 , 492 (1910).

[8] E. Rutherford, "The Origin of α and β Rays From Radioactive Substances," Philos. Mag. 24 , 453 (1912).

[9] H. Geiger and E. Marsden, "The Laws of Deflexion of α Particles Through Large Angles," Philos. Mag. 25 , 604 (1913).

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Key Questions

Rutherford's experiment showed that the atom does not contain a uniform distribution of charge.

Explanation:

Thomson's plum pudding model viewed the atom as a massive blob of positive charge dotted with negative charges.

A plum pudding was a Christmas cake studded with raisins ("plums"). So think of the model as a spherical Christmas cake.

When Rutherford shot α particles through gold foil, he found that most of the particles went through. Some scattered in various directions, and a few were even deflected back towards the source.

He argued that the plum pudding model was incorrect. The symmetrical distribution of charge would allow all the α particles to pass through with no deflection.

Rutherford proposed that the atom is mostly empty space. The electrons revolve in circular orbits about a massive positive charge at the centre.

His model explained why most of the α particles passed straight through the foil. The small positive nucleus would deflect the few particles that came close.

The nuclear model replaced the plum pudding model. The atom now consisted of a positive nucleus with negative electrons in circular orbits around it .

Rutherford Atomic Model

Definition of the rutherford model.

The Rutherford atomic model has 2 main parts: the nucleus, and the atom’s remaining space, occupied by electrons.

According to the model, the nucleus is a very small portion of the atom’s volume. It occupies a small space in the very center of the atom. Protons and neutrons make up the nucleus and define the atom’s chemical properties.

Rutherford also claimed in his model that electrons revolved around the nucleus in set orbits, like planets revolving around the Sun. This part of the theory was inaccurate, as explained in the last section.

Rutherford’s Gold Foil Experiment

The Rutherford gold foil experiment , also known as the scattering experiment, led to the creation of the model and explained the parts of the atom. In 1909, graduate student Ernest Marsden (under Ernest Rutherford’s supervision) fired alpha particles at a gold foil piece. Most of the particles passed directly through the foil, meaning that a majority of the space in each atom was unoccupied. However, a few particles were deflected, and some even backward. This must have been caused by tiny pockets of positive charge in the foil repelling the alpha particles back. Their discovery led to the creation of Rutherford’s model, in which the dense, positively-charged nucleus occupies a very small area in the center of each atom.

Shortcomings of the Rutherford Model

While common models today are based on the Rutherford atomic theory, it does not paint the complete picture:

• The model is missing parts and does not account for the location or distribution of electrons.
• Rutherford proposed that electrons orbit around the nucleus in set paths, but according to Maxwell’s theory , this is not possible because the atom would not be stable. Electromagnetic radiation from the electrons in orbit would cause the atom to collapse into the nucleus in 10 -8 seconds.
• Electrons increase and decrease energy levels randomly due to the acceleration and are not always in a standard circular orbit. They give off electromagnetic radiation due to the circular motion of orbiting; thus they must have some initial energy by the law of conservation of energy. The Rutherford atomic model does not account for the initial energy and subsequent energy level changes.

Further Reading

• Dalton’s Atomic Theory
• Bohr Atomic Model
• The Structure of an Atom

• Why Does Water Expand When It Freezes

Gold Foil Experiment

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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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Before Ernest Rutherford's landmark experiment with a few pieces of metal foil and alpha particles, the structure of the atom was thought to correspond with the plum pudding model. In summary, the plum pudding model was hypothesized by J.J. Thomson (the discoverer of the electron) who described an atom as being a large positively charged body that contained small, free–floating, negatively charged particles called electrons. The plum pudding model also states that the negative charge of the electrons is equivalent to the positive charge of the rest of the atom. The two charges cancel each other and cause the electrical charge of the atom to be zero (or neutral). The faulty aspect of this model is that it was constructed before the nucleus of an atom (and it's composition) was discovered, which is where Rutherford's research comes in.

In 1911, Ernest Rutherford conducted an experiment that proved that the mass of an atom is concentrated in the center (nucleus) of an atom. It also proved that an atom is mostly empty space.

When he shot a beam of alpha particles at a sheet of gold foil, a few of the particles were deflected. He concluded that a tiny, dense nucleus was causing the deflections.

Rutherford carried out a series of experiments using very thin foils of gold and other metals as targets for a particles from a radioactive source. They observed that the majority of particles penetrated the foil either undeflected or with only a slight deflection. But every now and then a particle was scattered (or deflected) at a large angle. In some instances, a particle actually bounced back in the direction from which it had come! This was a most surprising finding, in Thomson's model the positive charge of the atom was so diffuse that the positive a particles should have passed through the foil with very little deflection.

Rutherford was later able to explain the results of the α-scattering experiment in terms of a new model for the atom. According to Rutherford, most of the atom must be empty space. This explains why the majority of a particles passed through the gold foil with little or no deflection. The atom's positive charges, Rutherford proposed, are all concentrated in the nucleus, which is a dense central core within the atom. Whenever a particle came close to a nucleus in the scattering experiment, it experienced a large repulsive force and therefore a large deflection. Moreover, a particle traveling directly towards a nucleus would be completely repelled and its direction would be reversed.

Rutherford Explanations on gold foil experiment :

• Since most of the alpha particles pass straight through the gold foil without any deflection, it shows there is a lot of empty space in an atom.
• Those positively charged alpha particles deflected by large angles–some even backward, nearly in the direction from which they had come, which shows that there is a positive charge in center which is not distributed uniformly inside the atom.

• Around 1 in 8000 alpha particles were deflected by very large angles (over 90°), while the rest passed straight through with little or no deflection. From this, Rutherford concluded that the majority of the mass was concentrated in a central core.
• An atom has a tiny positively charged core (nucleus) which contains most of the mass over 99.9% of the mass of the atom.
• The electrons revolve around the nucleus in circular paths similar to the planets revolve around the Sun (solar system). The electrostatic force of attraction between the nucleus and electron provides centripetal force.
• He estimated that the radius of the nucleus was at least 1/100000 times smaller than that of the radius of the atom. Scientists imagined the size of the nucleus with the following similarity, if the size of the atom is that of Earth then the nucleus would have the size of an apple.
• The amount of positive charge in the nucleus is equal to the amount of negative charge on the electrons. So, the atom as a whole is electrically neutral.

One of the most important limitation of Rutherford model is that Rutherford's model failed to explain stability of atoms or why electrons which revolve around the nucleus do not lose energy and finally fall into the nucleus. Stability of atoms is explained by Bohr model of atom.

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• What is the model of the atom proposed by Ernest Rutherford?
• What were the results of Rutherford's experiment?
• What did Ernest Rutherford's atomic model get right and wrong?
• What was the impact of Ernest Rutherford's theory?
• When did science begin?
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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.

Related Questions

• 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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