understanding rutherford's 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

5 responses to “Gold Foil Experiment”

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

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

The gold foil experiments gave physicists their first view of the structure of the atomic nucleus and the physics underlying the everyday world.

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Additional Resources

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.

Here, an illustration of Rutherford's particle scattering device used in his gold foil experiment.

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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Experimental Evidence for the Structure of the Atom

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

A three-dimensional view of an apparatus similar to Geiger and Marsden's final cylindrical iteration, clearly showing the scattering of alpha particles by gold foil. (Source: )

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

A comparison between J.J. Thompson's "plum pudding" atomic model and the Rutherford model and its nucleus. Alpha particles and their scattering or lack thereof are depicted by the paths of the black arrows. (Source: )

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]

The schematics for the original two meter long tube that Geiger constructed and used to first detect the scattering of alpha particles by the atomic nucleus. At the point labeled R is the radon particle emission source, and Z the detector screen. (Source: )

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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Rutherford's Gold Foil Experiment

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 .

understanding rutherford's gold foil experiment

In 1909, Ernest Rutherford’s student reported some unexpected results from an experiment Rutherford had assigned him. Rutherford called this news the most incredible event of his life.

In the now well-known experiment, alpha particles were observed to scatter backwards from a gold foil. Rutherford’s explanation, which he published in May 1911, was that the scattering was caused by a hard, dense core at the center of the atom–the nucleus.

Ernest Rutherford was born in New Zealand, in 1871, one of 12 children. Growing up, he often helped out on the family farm, but he was a good student, and received a scholarship to attend the University of New Zealand. After college he won a scholarship in 1894 to become a research student at Cambridge. Upon receiving the news of this scholarship, Rutherford is reported to have said, “That’s the last potato I’ll ever dig.”

At Cambridge, the young Rutherford worked in the Cavendish lab with J.J. Thomson, discoverer of the electron. Rutherford’s talent was quickly recognized, and in 1898 he took a professorship at McGill University in Montreal. There, he identified alpha and beta radiation as two separate types of radiation, and studied some of their properties, though he didn’t know that alphas were helium nuclei. In 1901 Rutherford and chemist Frederick Soddy found that one radioactive element can decay into another. The discovery earned Rutherford the 1908 Nobel Prize in Chemistry, which irritated him somewhat because he considered himself a physicist, not a chemist. (Rutherford is widely quoted as having said, “All science is either physics or stamp collecting”)

In 1907 Rutherford returned to England, to the University of Manchester. In 1909, he and his colleague Hans Geiger were looking for a research project for a student, Ernest Marsden. Rutherford had already been studying the scattering of alpha particles off a gold target, carefully measuring the small forward angles through which most of the particles scattered. Rutherford, who didn’t want to neglect any angle of an experiment, no matter how unpromising, suggested Marsden look to see if any alpha particles actually scattered backwards.

Marsden was not expected to find anything, but nonetheless he dutifully and carefully carried out the experiment. He later wrote that he felt it was a sort of test of his experimental skills. The experiment involved firing alpha particles from a radioactive source at a thin gold foil. Any scattered particles would hit a screen coated with zinc sulfide, which scintillates when hit with charged particles. Marsden was to sit in the darkened room, wait for his eyes to adjust to the darkness, and then patiently stare at the screen, expecting to see nothing at all.

Instead, Marsden saw lots of tiny, fleeting flashes of yellowish light, on average more than one blip per second.

He could hardly believe what he saw. He tested and retested every aspect of the experiment, but when he couldn’t find anything wrong, he reported the results to Rutherford.

Rutherford too was astonished. As he was fond of saying, “It was as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

About one in every few thousand of the alpha particles fired at the gold target had scattered at an angle greater than 90 degrees. This didn’t fit with the prevailing model of the atom, the so-called plum pudding model developed by J.J. Thomson. In this model electrons were believed to be stuck throughout a blob of positively charged matter, like raisins in a pudding. But this sort of arrangement would only cause small angle scattering, nothing like what Marsden had observed.

After thinking about the problem for over a year, Rutherford came up with an answer. The only explanation, Rutherford suggested in 1911, was that the alpha particles were being scattered by a large amount of positive charge concentrated in a very small space at the center of the gold atom. The electrons in the atom must be orbiting around this central core, like planets around the sun, Rutherford proposed.

Rutherford carried out a fairly simple calculation to find the size of the nucleus, and found it to be only about 1/100,000 the size of the atom. The atom was mostly empty space.

In March 1911, Rutherford announced his surprising finding at a meeting of the Manchester Literary and Philosophical Society, and in May 1911, he published a paper on the results in the Philosophical Magazine .

Later Rutherford and Marsden tried the experiment with other elements as the target, and measured their nuclei as well.

The solar system model was not immediately accepted. One obvious problem was that according to Maxwell’s equations, electrons traveling in a circular orbit should radiate energy, and therefore slow down and fall into the nucleus. A solar system atom wouldn’t last long.

Fortunately, Niels Bohr was soon able to save the solar system model by applying new ideas from quantum mechanics. He showed that the atom could stay intact if electrons were only allowed to occupy certain discrete orbitals.

Though Rutherford still didn’t know what was in this nucleus he had discovered (protons and neutrons would be identified later), his insight in 1911, which overturned the prevailing plum pudding model of the atom, had opened the way for modern nuclear physics.

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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!

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Bohr Atomic Model
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Who was Ernest Rutherford?

Biography of Physicist 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 .

Rutherford's Nuclear World: The Story of the Discovery of the Nucleus | Young Rutherford | American Institute of Physics

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.

30.2 Discovery of the Parts of the Atom: Electrons and Nuclei

Learning objectives.

By the end of this section, you will be able to:

Describe how electrons were discovered.
Explain the Millikan oil drop experiment.
Describe Rutherford’s gold foil experiment.
Describe Rutherford’s planetary model of the atom.

Just as atoms are a substructure of matter, electrons and nuclei are substructures of the atom. The experiments that were used to discover electrons and nuclei reveal some of the basic properties of atoms and can be readily understood using ideas such as electrostatic and magnetic force, already covered in previous chapters.

Charges and Electromagnetic Forces

In previous discussions, we have noted that positive charge is associated with nuclei and negative charge with electrons. We have also covered many aspects of the electric and magnetic forces that affect charges. We will now explore the discovery of the electron and nucleus as substructures of the atom and examine their contributions to the properties of atoms.

The Electron

Gas discharge tubes, such as that shown in Figure 30.4 , consist of an evacuated glass tube containing two metal electrodes and a rarefied gas. When a high voltage is applied to the electrodes, the gas glows. These tubes were the precursors to today’s neon lights. They were first studied seriously by Heinrich Geissler, a German inventor and glassblower, starting in the 1860s. The English scientist William Crookes, among others, continued to study what for some time were called Crookes tubes, wherein electrons are freed from atoms and molecules in the rarefied gas inside the tube and are accelerated from the cathode (negative) to the anode (positive) by the high potential. These “ cathode rays ” collide with the gas atoms and molecules and excite them, resulting in the emission of electromagnetic (EM) radiation that makes the electrons’ path visible as a ray that spreads and fades as it moves away from the cathode.

Gas discharge tubes today are most commonly called cathode-ray tubes , because the rays originate at the cathode. Crookes showed that the electrons carry momentum (they can make a small paddle wheel rotate). He also found that their normally straight path is bent by a magnet in the direction expected for a negative charge moving away from the cathode. These were the first direct indications of electrons and their charge.

The English physicist J. J. Thomson (1856–1940) improved and expanded the scope of experiments with gas discharge tubes. (See Figure 30.5 and Figure 30.6 .) He verified the negative charge of the cathode rays with both magnetic and electric fields. Additionally, he collected the rays in a metal cup and found an excess of negative charge. Thomson was also able to measure the ratio of the charge of the electron to its mass, q e q e / m e / m e —an important step to finding the actual values of both q e q e and m e m e . Figure 30.7 shows a cathode-ray tube, which produces a narrow beam of electrons that passes through charging plates connected to a high-voltage power supply. An electric field E E is produced between the charging plates, and the cathode-ray tube is placed between the poles of a magnet so that the electric field E E is perpendicular to the magnetic field B B of the magnet. These fields, being perpendicular to each other, produce opposing forces on the electrons. As discussed for mass spectrometers in More Applications of Magnetism , if the net force due to the fields vanishes, then the velocity of the charged particle is v = E / B v = E / B . In this manner, Thomson determined the velocity of the electrons and then moved the beam up and down by adjusting the electric field.

To see how the amount of deflection is used to calculate q e / m e q e / m e , note that the deflection is proportional to the electric force on the electron:

But the vertical deflection is also related to the electron’s mass, since the electron’s acceleration is

The value of F F is not known, since q e q e was not yet known. Substituting the expression for electric force into the expression for acceleration yields

Gathering terms, we have

The deflection is analyzed to get a a , and E E is determined from the applied voltage and distance between the plates; thus, q e m e q e m e can be determined. With the velocity known, another measurement of q e m e q e m e can be obtained by bending the beam of electrons with the magnetic field. Since F mag = q e vB = m e a F mag = q e vB = m e a , we have q e / m e = a / vB q e / m e = a / vB . Consistent results are obtained using magnetic deflection.

What is so important about q e / m e q e / m e , the ratio of the electron’s charge to its mass? The value obtained is

This is a huge number, as Thomson realized, and it implies that the electron has a very small mass. It was known from electroplating that about 10 8 C/kg 10 8 C/kg is needed to plate a material, a factor of about 1000 less than the charge per kilogram of electrons. Thomson went on to do the same experiment for positively charged hydrogen ions (now known to be bare protons) and found a charge per kilogram about 1000 times smaller than that for the electron, implying that the proton is about 1000 times more massive than the electron. Today, we know more precisely that

where q p q p is the charge of the proton and m p m p is its mass. This ratio (to four significant figures) is 1836 times less charge per kilogram than for the electron. Since the charges of electrons and protons are equal in magnitude, this implies m p = 1836 m e m p = 1836 m e .

Thomson performed a variety of experiments using differing gases in discharge tubes and employing other methods, such as the photoelectric effect, for freeing electrons from atoms. He always found the same properties for the electron, proving it to be an independent particle. For his work, the important pieces of which he began to publish in 1897, Thomson was awarded the 1906 Nobel Prize in Physics. In retrospect, it is difficult to appreciate how astonishing it was to find that the atom has a substructure. Thomson himself said, “It was only when I was convinced that the experiment left no escape from it that I published my belief in the existence of bodies smaller than atoms.”

Thomson attempted to measure the charge of individual electrons, but his method could determine its charge only to the order of magnitude expected.

Since Faraday’s experiments with electroplating in the 1830s, it had been known that about 100,000 C per mole was needed to plate singly ionized ions. Dividing this by the number of ions per mole (that is, by Avogadro’s number), which was approximately known, the charge per ion was calculated to be about 1 . 6 × 10 − 19 C 1 . 6 × 10 − 19 C , close to the actual value.

An American physicist, Robert Millikan (1868–1953) (see Figure 30.8 ), decided to improve upon Thomson’s experiment for measuring q e q e and was eventually forced to try another approach, which is now a classic experiment performed by students. The Millikan oil drop experiment is shown in Figure 30.9 .

In the Millikan oil drop experiment, fine drops of oil are sprayed from an atomizer. Some of these are charged by the process and can then be suspended between metal plates by a voltage between the plates. In this situation, the weight of the drop is balanced by the electric force:

The electric field is produced by the applied voltage, hence, E = V / d E = V / d , and V V is adjusted to just balance the drop’s weight. The drops can be seen as points of reflected light using a microscope, but they are too small to directly measure their size and mass. The mass of the drop is determined by observing how fast it falls when the voltage is turned off. Since air resistance is very significant for these submicroscopic drops, the more massive drops fall faster than the less massive, and sophisticated sedimentation calculations can reveal their mass. Oil is used rather than water, because it does not readily evaporate, and so mass is nearly constant. Once the mass of the drop is known, the charge of the electron is given by rearranging the previous equation:

where d d is the separation of the plates and V V is the voltage that holds the drop motionless. (The same drop can be observed for several hours to see that it really is motionless.) By 1913 Millikan had measured the charge of the electron q e q e to an accuracy of 1%, and he improved this by a factor of 10 within a few years to a value of − 1 . 60 × 10 − 19 C − 1 . 60 × 10 − 19 C . He also observed that all charges were multiples of the basic electron charge and that sudden changes could occur in which electrons were added or removed from the drops. For this very fundamental direct measurement of q e q e and for his studies of the photoelectric effect, Millikan was awarded the 1923 Nobel Prize in Physics.

With the charge of the electron known and the charge-to-mass ratio known, the electron’s mass can be calculated. It is

Substituting known values yields

where the round-off errors have been corrected. The mass of the electron has been verified in many subsequent experiments and is now known to an accuracy of better than one part in one million. It is an incredibly small mass and remains the smallest known mass of any particle that has mass. (Some particles, such as photons, are massless and cannot be brought to rest, but travel at the speed of light.) A similar calculation gives the masses of other particles, including the proton. To three digits, the mass of the proton is now known to be

which is nearly identical to the mass of a hydrogen atom. What Thomson and Millikan had done was to prove the existence of one substructure of atoms, the electron, and further to show that it had only a tiny fraction of the mass of an atom. The nucleus of an atom contains most of its mass, and the nature of the nucleus was completely unanticipated.

Another important characteristic of quantum mechanics was also beginning to emerge. All electrons are identical to one another. The charge and mass of electrons are not average values; rather, they are unique values that all electrons have. This is true of other fundamental entities at the submicroscopic level. All protons are identical to one another, and so on.

The Nucleus

Here, we examine the first direct evidence of the size and mass of the nucleus. In later chapters, we will examine many other aspects of nuclear physics, but the basic information on nuclear size and mass is so important to understanding the atom that we consider it here.

Nuclear radioactivity was discovered in 1896, and it was soon the subject of intense study by a number of the best scientists in the world. Among them was New Zealander Lord Ernest Rutherford, who made numerous fundamental discoveries and earned the title of “father of nuclear physics.” Born in Nelson, Rutherford did his postgraduate studies at the Cavendish Laboratories in England before taking up a position at McGill University in Canada where he did the work that earned him a Nobel Prize in Chemistry in 1908. In the area of atomic and nuclear physics, there is much overlap between chemistry and physics, with physics providing the fundamental enabling theories. He returned to England in later years and had six future Nobel Prize winners as students. Rutherford used nuclear radiation to directly examine the size and mass of the atomic nucleus. The experiment he devised is shown in Figure 30.10 . A radioactive source that emits alpha radiation was placed in a lead container with a hole in one side to produce a beam of alpha particles, which are a type of ionizing radiation ejected by the nuclei of a radioactive source. A thin gold foil was placed in the beam, and the scattering of the alpha particles was observed by the glow they caused when they struck a phosphor screen.

Alpha particles were known to be the doubly charged positive nuclei of helium atoms that had kinetic energies on the order of 5 MeV 5 MeV when emitted in nuclear decay, which is the disintegration of the nucleus of an unstable nuclide by the spontaneous emission of charged particles. These particles interact with matter mostly via the Coulomb force, and the manner in which they scatter from nuclei can reveal nuclear size and mass. This is analogous to observing how a bowling ball is scattered by an object you cannot see directly. Because the alpha particle’s energy is so large compared with the typical energies associated with atoms ( MeV MeV versus eV eV ), you would expect the alpha particles to simply crash through a thin foil much like a supersonic bowling ball would crash through a few dozen rows of bowling pins. Thomson had envisioned the atom to be a small sphere in which equal amounts of positive and negative charge were distributed evenly. The incident massive alpha particles would suffer only small deflections in such a model. Instead, Rutherford and his collaborators found that alpha particles occasionally were scattered to large angles, some even back in the direction from which they came! Detailed analysis using conservation of momentum and energy—particularly of the small number that came straight back—implied that gold nuclei are very small compared with the size of a gold atom, contain almost all of the atom’s mass, and are tightly bound. Since the gold nucleus is several times more massive than the alpha particle, a head-on collision would scatter the alpha particle straight back toward the source. In addition, the smaller the nucleus, the fewer alpha particles that would hit one head on.

Although the results of the experiment were published by his colleagues in 1909, it took Rutherford two years to convince himself of their meaning. Like Thomson before him, Rutherford was reluctant to accept such radical results. Nature on a small scale is so unlike our classical world that even those at the forefront of discovery are sometimes surprised. Rutherford later wrote: “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 backwards ... [meant] ... the greatest part of the mass of the atom was concentrated in a tiny nucleus.” In 1911, Rutherford published his analysis together with a proposed model of the atom. The size of the nucleus was determined to be about 10 − 15 m 10 − 15 m , or 100,000 times smaller than the atom. This implies a huge density, on the order of 10 15 g/cm 3 10 15 g/cm 3 , vastly unlike any macroscopic matter. Also implied is the existence of previously unknown nuclear forces to counteract the huge repulsive Coulomb forces among the positive charges in the nucleus. Huge forces would also be consistent with the large energies emitted in nuclear radiation.

The small size of the nucleus also implies that the atom is mostly empty inside. In fact, in Rutherford’s experiment, most alphas went straight through the gold foil with very little scattering, since electrons have such small masses and since the atom was mostly empty with nothing for the alpha to hit. There were already hints of this at the time Rutherford performed his experiments, since energetic electrons had been observed to penetrate thin foils more easily than expected. Figure 30.11 shows a schematic of the atoms in a thin foil with circles representing the size of the atoms (about 10 − 10 m 10 − 10 m ) and dots representing the nuclei. (The dots are not to scale—if they were, you would need a microscope to see them.) Most alpha particles miss the small nuclei and are only slightly scattered by electrons. Occasionally, (about once in 8000 times in Rutherford’s experiment), an alpha hits a nucleus head-on and is scattered straight backward.

Based on the size and mass of the nucleus revealed by his experiment, as well as the mass of electrons, Rutherford proposed the planetary model of the atom . The planetary model of the atom pictures low-mass electrons orbiting a large-mass nucleus. The sizes of the electron orbits are large compared with the size of the nucleus, with mostly vacuum inside the atom. This picture is analogous to how low-mass planets in our solar system orbit the large-mass Sun at distances large compared with the size of the sun. In the atom, the attractive Coulomb force is analogous to gravitation in the planetary system. (See Figure 30.12 .) Note that a model or mental picture is needed to explain experimental results, since the atom is too small to be directly observed with visible light.

Rutherford’s planetary model of the atom was crucial to understanding the characteristics of atoms, and their interactions and energies, as we shall see in the next few sections. Also, it was an indication of how different nature is from the familiar classical world on the small, quantum mechanical scale. The discovery of a substructure to all matter in the form of atoms and molecules was now being taken a step further to reveal a substructure of atoms that was simpler than the 92 elements then known. We have continued to search for deeper substructures, such as those inside the nucleus, with some success. In later chapters, we will follow this quest in the discussion of quarks and other elementary particles, and we will look at the direction the search seems now to be heading.

PhET Explorations

Rutherford scattering.

How did Rutherford figure out the structure of the atom without being able to see it? Simulate the famous experiment in which he disproved the Plum Pudding model of the atom by observing alpha particles bouncing off atoms and determining that they must have a small core.

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That actually explain what's on your next test, gold foil experiment, from class:, intro to chemistry.

The gold foil experiment, also known as the Rutherford scattering experiment, was a groundbreaking scientific investigation that provided crucial evidence for the existence of the atomic nucleus and helped shape our understanding of atomic structure.

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5 Must Know Facts For Your Next Test

The gold foil experiment was conducted by Ernest Rutherford and his colleagues in 1911, building upon the earlier work of J.J. Thomson and the discovery of the electron.
In the experiment, a beam of positively charged alpha particles was directed at a thin sheet of gold foil, and the scattering of these particles was observed.
Contrary to the prevailing model of the atom at the time, which envisioned the atom as a uniform, positively charged sphere, the gold foil experiment revealed that most of the atom's mass and positive charge was concentrated in a tiny, dense nucleus.
The unexpected observation of some alpha particles being deflected at large angles, or even bouncing back, led Rutherford to conclude that the atom must have a dense, positively charged core surrounded by a mostly empty space.
The gold foil experiment provided the foundation for Rutherford's model of the atom, which proposed that the atom consists of a small, dense nucleus orbited by lightweight electrons.

Review Questions

The gold foil experiment was a pivotal moment in the development of atomic theory. It challenged the prevailing model of the atom as a uniform, positively charged sphere and provided evidence for the existence of a dense, positively charged nucleus within the atom. Rutherford's observations of the unexpected scattering of alpha particles led him to propose a new model of the atom, with the majority of the atom's mass and positive charge concentrated in a small, central nucleus. This was a crucial step in the evolution of atomic theory, as it paved the way for the modern understanding of the atom's structure and the role of the nucleus.
In the gold foil experiment, a beam of positively charged alpha particles was directed at a thin sheet of gold foil. Contrary to expectations, most of the alpha particles passed through the foil undeflected, while a small percentage were deflected at large angles, and some even bounced back. These observations led Rutherford to conclude that the atom must have a dense, positively charged core surrounded by a mostly empty space. The unexpected scattering of the alpha particles, particularly the large-angle deflections and the occasional backscattering, provided the crucial evidence that the atom could not be a uniform, positively charged sphere as previously thought, but rather had a concentrated, dense nucleus at its center.
The gold foil experiment was a pivotal moment in the evolution of atomic theory, as it challenged the prevailing model of the atom and provided the foundation for Rutherford's model of the atom. The unexpected scattering of the alpha particles, with some being deflected at large angles or even bouncing back, led Rutherford to conclude that the atom must have a dense, positively charged nucleus surrounded by a mostly empty space. This was a significant departure from the earlier model of the atom as a uniform, positively charged sphere. Rutherford's model, which proposed that the majority of the atom's mass and positive charge was concentrated in a small, central nucleus, paved the way for the development of the modern atomic model, which includes the concept of the nucleus, the arrangement of electrons around the nucleus, and the discovery of protons and neutrons as the constituents of the nucleus. The gold foil experiment, therefore, played a crucial role in shaping our understanding of the atom's internal structure and the behavior of subatomic particles.

Related terms

Atomic Model : A representation of the internal structure and behavior of an atom, which has evolved over time through scientific discoveries and experiments.

Atomic Nucleus : The dense, central core of an atom, containing protons and neutrons, which accounts for the majority of the atom's mass.

Rutherford Scattering : The deflection of charged particles, such as alpha particles, when they pass through thin metal foils, which was observed and explained by Ernest Rutherford.

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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 .

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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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.