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The Geiger - Marsden Experiment

We get a planetary view of the atom nucleus 1/10,000 atoms diameter 99.9% of atoms mass is in the nucleus but..... an orbiting electron must be accelerating ..why – powerpoint ppt presentation.

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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.

J.J. Thomson model of the atom

Gold foil experiments, rutherford model of the atom.

• The real atomic model

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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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The rutherford-geiger-marsden experiment.

April 11, 2017 Alpha Spectroscopy , English Posts 85,372 Views

What made by Rutherford and his assistants Geiger and Marsden is perhaps one of the most important experiments of nuclear physics.

The experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.

In the experiment, Rutherford sent a beam of alpha particles (helium nuclei) emitted from a radioactive source against a thin gold foil (the thickness of about 0.0004 mm, corresponding to about 1000 atoms).

Surrounding the gold foil it was placed a zinc sulfide screen that would show a small flash of light when hit by a scattered alpha particle. The idea was to determine the structure of the atom and understand if it were what supposed by Thomson (atom without a nucleus, also known as pudding model ) or if there was something different.

In particular, if the atom had an internal nucleus separated from external electrons, then they would have been able to observe events, or particles, with large angle of deviation . Obtained, actually, these results, the New Zealand physicist concluded that the atom was formed by a small and compact nucleus , but with high charge density, surrounded by an electron cloud. In the image below it is depicted the interaction of the alpha particles beam with the nuclei of the thin gold foil; one can see how the majority of the particles passes undisturbed, or with small angles of deflection, through the “empty” atom, some particles, however, passing close to the nucleus are diverted with a high angle or even bounced backwards.

The interaction between an alpha particle and the nucleus (elastic collision) is also known as Coulomb scattering , because the interaction in the collision is due to the Coulomb force. In the diagram below it is shown the detail of the interaction between an alpha particle and the nucleus of an atom.

Experimental Setup

In the PhysicsOpenLab “laboratory” we tried to replicate the famous Rutherford experiment. With the equipment already used in alpha spectroscopy we built a setup based on an alpha solid-state detector , a 0.9 μCi Am 241 source and a gold foil as a scatterer. In these post we describe the equipment used : Alpha Spectrometer , Gold Leaf Thickness  . The main purpose is not to make precision measurements but to make a qualitative assessment of the scattering as a function of deflection. The images below show the experimental setup:

The alpha source is actually 0.9 μCi of Am 241 (from smoke detector) which emits alpha particles with energy of 5.4 MeV. The alpha particle beam is collimated by a simple hole in a wooden screen. Source and collimator are fixed on a arm free to rotate around a pivot, which hosts the gold foil that acts as a scatterer. The whole is placed inside a sealed box that acts as a vacuum chamber with the help of an ordinary oil rotary vacuum pump. The images below show the “vacuum chamber” and the electronic part for amplification and acquisition connected to the PC for counting events.

Linear Scale :

Semilog Scale

The results obtained in our experiment approach, albeit with obvious limitations, to the expected theoretical results, represented in the following graph:

For completeness, we report also at the side the formula that describes the distribution of the number of the counted particles in function of the scattering angle. Interestingly, this depends on the power of two the atomic number of the target and is inversely proportional to the fourth power of the sin (θ/2).

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Gamma Spectroscopy with KC761B

Abstract: in this article, we continue the presentation of the new KC761B device. In the previous post, we described the apparatus in general terms. Now we mainly focus on the gamma spectrometer functionality.

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Gold foil experiment notes and diagrams

Subject: Physics

Age range: 14-16

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Notes with diagrams and video links on the Rutherford/Geiger/Marsden gold foil experiment. Explains the plum pudding model, why they used alpha particles, what Rutherford expected and finishes with a bullet point list of features of the modern view of the atom.

Covering one page, the notes are suitable for GCSE physics and particularly targeted at AQA GCSE physics. It could also be used for OCR Gateway GCSE chemistry and physics and Edexcel 9-1 physics.

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Lesson idea. This presentation diverges from pure astronomy to cover the work of scientists in understanding our universe. Students possibly believe that we have always known what the world is made from. And, even that we actually know now! But these slides tell how theories of the atom developed over many years. An audio file fills in detail. (The next in this series considers how gases interact in the gas laws).

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Nuclear Physics

Mar 15, 2019

480 likes | 617 Views

Nuclear Physics. The famous Geiger-Marsden Alpha scattering experiment (under Rutherford’s guidance). In 1909, Geiger and Marsden were studying how alpha particles are scattered by a thin gold foil. Thin gold foil. Alpha source. Geiger-Marsden.

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• energy levels
• kinetic energy
• alpha particles
• nuclear energy levels
• energy levels exist inside

Presentation Transcript

The famous Geiger-Marsden Alpha scattering experiment (under Rutherford’s guidance) In 1909, Geiger and Marsden were studying how alpha particles are scattered by a thin gold foil. Thin gold foil Alpha source

Geiger-Marsden As expected, most alpha particles were detected at very small scattering angles Thin gold foil Small-angle scattering Alpha particles

Geiger-Marsden To their great surprise, they found that some alpha particles (1 in 20 000) had very large scattering angles Thin gold foil Small-angle scattering Alpha particles Large-angle scattering

Explaining Geiger and Marsdens’ results The results suggested that the positive (repulsive) charge must be concentrated at the centre of the atom. Most alpha particles do not pass close to this so pass undisturbed, only alpha particles passing very close to this small nucleus get repelled backwards (the nucleus must also be very massive for this to happen). Remember on this scale, if the nucleus is 2 cm wide, the atom would be 200 m wide!

Rutherford did the calculations! Rutherford calculated theoretically the number of alpha particles that should be scattered at different angles (using Coulomb’s law). He found agreement with the experimental results if he assumed the atomic nucleus was confined to a diameter of about 10-15 metres.

Closest approach Using the idea of energy conservation, it is possible to calculate the closest an alpha particle could get to the nucleus during a head-on collision. Alpha particle nucleus

Closest approach Initially, the alpha particle has kinetic energy = ½mu2 K.E. = ½mu2

Closest approach At the point of closest approach, the particle reaches a distance b from the nucleus and comes momentarily to rest. K.E. = 0 b

Closest approach All the initial kinetic energy has been transformed to electrical potential energy. K.E. = 0 b

Closest approach Using the formula for electrical potential energy which is derived from Coulomb’s law Kinetic energy lost = Electrical potential ½mu2 = 1 q1q2 4πεo b K.E. = 0 b

Closest approach Rearranging we get; b = 1 q1q2 4πεo½mu2 K.E. = 0 b

Closest approach For an alpha particle, m = 6.7 x 10-27 kg, q1 = 2 x (1.6 x 10-19 C) and u is around 2 x 107 m.s-1. If the foil is made of gold, q2 is 79 x (1.6 x 10-19 C). b = 1 q1q2 4πεo½mu2 b = 1 x (2 x 1.6 x 10-19 C) x (79 x 1.6 x 10-19 C) 4π x 8.854 x 10-12 Fm-1½ x 6.7 x 10-27 kg x (2 x 107 m.s-1)2 b = 2.7 x 10-14 m

The mass spectrometer A VERY useful machine for measuring the masses of atoms (ions) and their relative abundances. What is it and how does it work Mr Porter?

The Mass Spectrometer Region of magnetic field velocity selector ions produced ions accelerated ion beam detector

The Mass Spectrometer Substance to be tested is turned into a gas by heating. Electrons are produced at a hot cathode and accelerated by an electric field. Collision with gas particles produces ions.

The Mass Spectrometer Ions acclerated by an electric field

The Mass Spectrometer Ions enter the velocity selector which contains an electric field (up the page) and magnetic field (into the page) at right angles to each other. By choosing a suitable value for the magnetic field the ions continue in a straight line. i.e. The force produced by the electric filed (eE) is equal to the force produced by the magnetic field (Bev). eE = Bev

The Mass Spectrometer eE = Bev or v = E/B This means that only ions with a specific velocity pass through this region. (Hence ”velocity selector”)

The Mass Spectrometer The selected ions all with the same velocity (but different masses of course) enter the second region of magnetic field (also into the page). They are deflected in a circular path.

The Mass Spectrometer Heavier ions continue forward and hit the sides, as do ions that are too light. Only ions of one particluar mass reach the detector. The radius of the circle is given by R = mv/eB so the mass can be calculated from m = ReB/v

The Mass Spectrometer The magnetic field can be varied so that ions of different mass can be detected (higher B would mean that ions of larger mass could be directed at the detector).

The Mass Spectrometer The detector can measure the numbers of ions detected, hence giving an idea of relative abundance of different ions.

The Mass Spectrometer The mass spectrometer is particulary useful for identifying isotopes of the same element.

Nuclear energy levels We have seen previously that electrons exist in specific energy levels around the atom. There is evidence that energy levels exist inside the nucleus too. Wow!

Nuclear energy levels When a nucleus decays by emitting an alpha particle or a gamma ray, the particles or photons emitted are only at specific energies (there is not a complete range of energies emitted, only certain specific levels).

Nuclear energy levels An alpha particle or photon thus has an energy equal to the difference between energy levels of the nucleus. 51.57 energy levels in 235U (MeV) 0.051 0.013 0.000

Nuclear energy levels In the alpha decay of 239Pu to 235U, the plutonium nucleus with an energy of 51.57 MeV can decay into Uranium at 3 different energy levels. 51.57 energy levels in 235U (MeV) Plutonium-239 0.051 0.013 0.000

Nuclear energy levels If the 239Pu (51.57 MeV) decays to the ground state of 235U (0 MeV), an alpha particle of energy 51.57 MeV is emitted. 51.57 energy levels in 235U (MeV) Plutonium-239 Alpha emission (51.57 MeV) 0.051 0.013 0.000

Alpha emission (51.52 MeV) Gamma emission (0.051 MeV) Nuclear energy levels If the 239Pu (51.57 MeV) decays to the 2nd excited state of 235U (0.051 MeV), an alpha particle of energy 51.57-0.051 = 51.52 MeV is emitted. The uranium nucleus is now in an excited state so can decay further by gamma emission to the ground state. 51.57 energy levels in 235U (MeV) Plutonium-239 0.051 0.013 0.000

Alpha emission (51.52 MeV) Gamma emission (0.038 MeV) Nuclear energy levels In fact the nucleus could decay first to the 0.013 level, and then the ground state, thus emitting two gamma photons. 51.57 energy levels in 235U (MeV) Plutonium-239 0.051 0.013 Gamma emission (0.013MeV) 0.000

Radioactive decay Beta (β) and positron decay (β+) decay Positron? That sounds interesting, what is it?

Radioactive decay In beta decay, a neutron in the nucleus decays into a proton, an electron and an antineutrino. n p + e + ve 1 0 1 1 0 -1 0 0

Radioactive decay In positron decay, a proton in the nucleus decays into a neutron, a positron (the antiparticle of the electron) and a neutrino. p n + e + ve 1 1 1 0 0 +1 0 0

Radioactive decay We must not think that the decaying particle actually consists of the three particles in which it splits.

The antineutrino The antineutrino in beta decay was not detected until 1953, although its presence had been predicted theoretically. n p + e + ve 1 0 1 1 0 -1 0 0

The antineutrino The mass of the neutron is bigger than that of the proton and electron together. n p + e + ve 1.008665u – (1.007276 + 0.0005486)u = 0.00084u 1 0 1 1 0 -1 0 0 1.008665 u 1.007276 u 0.0005486 u

The antineutrino This corresponds (using E = mc2) to an energy of 0.783 MeV. n p + e + ve 1 0 1 1 0 -1 0 0

The antineutrino This extra energy should show up as kinetic energy of the products (proton and electron). Since the electron should carry most of the kinetic energy away, so we should observe electrons with an energy of about 0.783 MeV. n p + e + ve In fact we observe electrons with a range of energies from zero up to 0.783 MeV. 1 0 1 1 0 -1 0 0

The antineutrino Where is the missing energy? In 1933 Wolfgang Pauli and Enrico Fermi hypothesized the existence of a third very light particle produced during the decay. Enrico Fermi coined the term neutrino for the ”little neutral one” n p + e + ve 1 0 1 1 0 -1 0 0 Ahhhhh! The little neutral one!

The radioactive decay law If the number of nuclei present in a sample at t = 0 is N0, the number N still present at time t later is given by N = Noe-λt where λ is the decay constant (the probability that a nucleus will decay in unit time)

The radioactive decay law N = Noe-λt No Number of original nuclei present time

Half-life After one half-life, the number of original nuclei present is equal to N0/2. Putting this into the radioactive decay law; N0/2 = N0e(-λt½) where t½ is the half-life

Half-life N0/2 = N0e(-λt½) taking logarithms we find λt½ = ln2 λt½ = 0.693 This is the relationship between the decay constant and the half-life.

Measuring half-life For short half-lives, the half life can usually be measured directly.

Measuring half-life For longer half lifes, values of activity can be measured and the decay law can be used to calculate λ and thus t½. Measure the activity A and chemically find the number of atoms of the isotope. Use A = λN and then λt½ = ln2

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Browse Course Material

Course info.

• Prof. Donald Sadoway

Departments

• Materials Science and Engineering

As Taught In

• Chemical Engineering

Learning Resource Types

Introduction to solid state chemistry, 3. atomic models: rutherford & bohr.

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Session Overview

Prerequisites

Before starting this session, you should be familiar with:

• Session 2: The Periodic Table

Looking Ahead

Prof. Sadoway discusses the atomic spectra of hydrogen ( Session 4 ).

Learning Objectives

After completing this session, you should be able to:

• Understand Thomson’s “plum pudding” model .
• Understand Rutherford’s “nuclear” model .
• Explain the Bohr model of hydrogen .
• Understand Bohr’s quantization condition.

Archived Lecture Notes #1 (PDF) , Sections 1-3

Lecture Video

• Download video
• Download transcript

Lecture Slides (PDF - 9.3MB)

Periodic Table and Table of Constants

Lecture Summary

Prof. Sadoway talks about the principles of modern chemistry and how that led to the understanding of the structure of the atom . He details Bohr’s postulates for the hydrogen atom and discusses how the Planck-Einstein relationship applies to electron transitions. He defines the different isotopes of hydrogen.

This lecture includes the following:

• Electrons are distributed uniformly throughout the atom
• Conclusions from the gold foil experiment
• Majority of the mass is found in the nucleus
• Electrons orbit around the nucleus
• Explanation of blackbody radiation and atomic spectra
• Electrons follow circular orbits around a nucleus
• Orbital angular momentum is quantized hence only certain orbits are possible
• Electrons in stable orbits do not radiate
• Electrons change orbits by radiating or absorbing photons

Problems (PDF)

Solutions (PDF)

Textbook Problems

For Further Study

Supplemental readings.

Ottaviani, J. Suspended in Language: Niels Bohr’s Life, Discoveries, and the Century He Shaped . GT Labs: Ann Arbor, MI, 2004. ISBN: 9780978803728.

Rozental, S. Niels Bohr: His Life and Work as Seen by His Friends and Colleagues . New York, NY: Wiley, 1967.

Bohr, Niels H. D. On the Constitution of Atoms and Molecules . New York, NY: W.A. Benjamin, 1963.

Bohr, Niels H. D. Atomic Physics and Human Knowledge . New York, NY: Wiley, 1958.

Bohr, Niels. “ On the Constitution of Atoms and Molecules. ” Philosophical Magazine Series 6 26 (July 1913): 1-15.

Cathcart, B. The Fly in the Cathedral: How a Small Group of Cambridge Scientists Won the Race to Split the Atom . New York, NY: Penguin, 2005. ISBN: 9780670883219.

Andrade, E. N. Rutherford and the Nature of the Atom . Garden City, NY: Doubleday, 1964.

Frayn, M. Copenhagen: A Play in Two Acts . New York, NY: S. French, 2000.

Miller, D. P. Discovering Water: James Watt, Henry Cavendish and the Nineteenth Century Water Controversy . Burlington, VT: Ashgate, 2004. ISBN: 9780754631774.

Cavendish Laboratory

How Atoms Work

Joseph Thompson - 1906 Nobel Prize in Physics

Ernest Rutherford - 1908 Nobel Prize in Chemistry

Johannes Geiger

Ernest Marsden

Max Planck - 1918 Nobel Prize in Physics

Albert Einstein - 1921 Nobel Prize in Physics

Niels Bohr - 1922 Nobel Prize in Physics

Robert Millikan - 1923 Nobel Prize in Physics

Henry Cavendish

Werner Heisenberg - 1932 Nobel Prize in Physics

Harold Urey - 1934 Nobel Prize in Chemistry

Charles-Augustin de Coulomb

James Prescott Joule

Other OCW and OER Content

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Geiger-Marsden's Gold Foil Experiment & Rutherford's Model of the Atom

Hsc physics syllabus.

investigate, assess and model the experimental evidence supporting the nuclear model of the atom, including:

assess the limitations of the Rutherford and Bohr atomic models

Geiger-Marsden's Gold Foil Experiment

Geiger and Marsden performed an experiment using a thin gold foil to investigate the structure of the atom.

Rutherford’s Model of the Atom

Rutherford's model of the atom is characterised by a few key features:

• a highly concentrated positively charged region in the centre of the atom, called the nucleus
• most of the atom is empty space
• electrons orbit the nucleus

Geiger and Marsden fired alpha particles (helium nuclei) at a thin gold foil. The gold foil was surrounded by a screen that would cause scintillations when alpha particles hit it. 

What Did the Gold Foil Experiment Show?

• Observation 1: Most alpha particles passed through the gold foil undeflected as most scintillations were observed directly behind the gold foil. 

This observation supported Rutherford's postulate that an atom is mostly empty space. 

• Observation 2: A few alpha particles were deflected and some of which were reflected back (large angle deflections). The angle of deflection was measured by the position at which they were detected on the fluorescent screen.

This observation supported the presence of a region in the atom of highly concentrated positive charge (nucleus). In an atom of gold, the charge and mass of the nucleus are substantially greater than that of an alpha particles. As a result, when an alpha particle collided with the nucleus, it was reflected.

Geiger and Marsden's gold foil experiment not only provided evidence for Rutherford's model of the atom, it rejected the preceding atomic model proposed by Thomson . Although Thomson's model predicted that all, if not most, alpha particles would pass through the gold foil undeflected, it could not account for the few alpha particles that were reflected. 

How was the Proton Discovered?

After Geiger and Marsden's gold foil experiment, Rutherford tried to investigate the content of the nucleus. 

Rutherford fired alpha particles at a sample of nitrogen gas, which resulted in a transmutation reaction producing protons. 

Rutherford conducted a similar experiment as Thomson to determine the value of the charge to mass ratio of a proton. He showed that a proton is positively charged and much heavier than an electron. 

Although Rutherford demonstrated that the charge of proton(s) accounts for the positive nature of the nucleus, he wasn't able to account for the nuclear mass.

Limitations of Rutherford's Model of the Atom

There are three main limitations to Rutherford's atomic model:

1. The model fails to explain the stability of electrons' orbital motion.

Rutherford proposed that electrons must be orbiting the positively charged nucleus like how satellites orbit the Earth, otherwise they would clash into the nucleus due to the electrostatic attraction towards the nucleus. However, by Maxwell's electromagnetic theory, electrons should emit radiation when they experience centripetal acceleration. By the law of conservation of energy, an electron's kinetic energy should gradually decrease, leading to its spiralling motion into the nucleus. 

Previous section: Millikan's Oil Drop Experiment

Next section: Chadwick's Discovery of The Neutron

BACK TO MODULE 8: FROM THE UNIVERSE TO THE ATOM

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Syllabus Edition

First teaching 2014

Last exams 2024

Discovery of the Nucleus ( DP IB Physics: SL )

Revision note.

The Rutherford-Geiger-Marsden Experiment

• Evidence for the structure of the atom was discovered by Ernest Rutherford in the beginning of the 20th century from the study of α-particle scattering
• The experimental setup consists of alpha particles fired at thin gold foil and a detector on the other side to detect how many particles deflected at different angles

α-particle scattering experiment set up

• α-particles are the nucleus of a helium atom and are positively charged

When α-particles are fired at thin gold foil, most of them go straight through but a small number bounce straight back

• From this experiment, Rutherford results were:
• This suggested the atom is mainly empty space
• This suggested there is a positive nucleus at the centre (since two positive charges would repel)
• This suggested the nucleus is extremely small and this is where the mass and charge of the atom is concentrated
• It was therefore concluded that atoms consist of small dense positively charged nuclei, surrounded by negatively charged electrons

An atom: a small positive nucleus, surrounded by negative electrons

• (Note: The atom is around 100,000 times larger than the nucleus!)

Worked example

     ANSWER:   A

• The Rutherford scattering experience directed parallel beams of α-particles at gold foil
• Most of the α-particles went straight through the foil
• The largest value of n will therefore be at small angles
• Some of the α-particles were deflected through small angles
• n drops quickly with increasing angle of deflection θ
• These observations fit with graph A

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