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The Geiger-Marsden Experiment

Establishing the Nuclear Structure of the Atom

CSEC Physics: The Birth of Nuclear Physics

Essential Understanding: The Geiger-Marsden experiment, conducted between 1908-1911, revolutionized our understanding of atomic structure. By firing alpha particles at thin metal foils, Hans Geiger and Ernest Marsden made discoveries that led Ernest Rutherford to propose the nuclear model of the atom—one of the most important breakthroughs in the history of physics.

🔬 Key Experiment: Alpha Particle Scattering
📈 Exam Focus: Rutherford's Nuclear Model
🎯 Problem Solving: Scattering Observations

Historical Context: Before the Experiment

Before the Geiger-Marsden experiment, the prevailing model of the atom was J.J. Thomson's "Plum Pudding" model, proposed in 1897 after his discovery of the electron. According to this model:

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Thomson's Plum Pudding Model (1897)

The atom consisted of:

  • A diffuse sphere of positive charge
  • Negatively charged electrons embedded within it
  • The positive and negative charges balanced, making atoms neutral

Key Limitation: The positive charge was spread throughout the atom like "dough," with no concentrated nucleus.

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The Big Question

Scientists wanted to understand:

  • How is positive charge distributed in the atom?
  • Where are the electrons located?
  • Is the positive charge concentrated or diffuse?

The Solution: Use alpha particles (positively charged) to probe the atom's structure like tiny bullets!

The Scientists Behind the Experiment

Hans Geiger (1882-1945)

German physicist who worked with Rutherford at the University of Manchester. He developed the Geiger counter for detecting radiation and conducted the famous scattering experiments with Marsden.

Ernest Marsden (1889-1970)

New Zealand-born physicist who was just 20 years old when he conducted the gold foil experiment under Rutherford's direction. He later became a prominent scientist in New Zealand.

Ernest Rutherford (1871-1937)

The "father of nuclear physics." Although he did not personally conduct the gold foil experiment, he directed the research and interpreted the results, proposing the revolutionary nuclear model in 1911.

The Experiment: Setup and Procedure

The Geiger-Marsden Experimental Setup (1908-1909) Alpha Source (RaBr) Collimator Evacuated Chamber Gold Foil ZnS Screen (detector) Scope Straight through (~98%) Scattered Reflected! Observed Results • ~98% passed straight • ~2% deflected • ~1 in 8000 reflected back

Key Components of the Apparatus

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Alpha Particle Source

Material: Radium bromide (RaBr) or polonium

Properties of Alpha Particles:

  • Helium nuclei (2 protons + 2 neutrons)
  • Positive charge (+2e)
  • Relatively heavy mass
  • High energy, fast-moving

Why Alpha Particles? Their positive charge would interact with positive charge in atoms, and their mass made them effective probes.

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Evacuated Chamber

Purpose: The chamber was pumped vacuum to prevent alpha particles from colliding with air molecules.

Why Vacuum?

  • Alpha particles have short range in air (~7cm)
  • Collisions with air would scatter them randomly
  • Only interactions with gold atoms would be observed

Result: Clear, accurate scattering patterns from the gold foil only.

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Thin Gold Foil

Thickness: Approximately 400 atoms thick (0.00004 cm)

Why Gold?

  • Highly malleable - can be made extremely thin
  • High atomic number (79 protons) - strong scattering
  • Uniform thickness achievable

The Idea: If Thomson's model was correct, alpha particles should pass through with minimal deflection.

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Zinc Sulfide Screen + Microscope

Detection Method: When alpha particles struck the ZnS screen, they produced tiny flashes of light (scintillations).

Observations Made:

  • Geiger and Marsden counted flashes at different angles
  • They moved the detector around the foil
  • Every flash represented a scattered alpha particle

Breakthrough: Some particles scattered at HUGE angles!

The Surprising Results

When Geiger and Marsden analyzed their data, the results were completely unexpected and overturned the Plum Pudding model entirely. Here is what they observed:

Geiger-Marsden Results: Alpha Particle Scattering Expected: Diffuse positive charge Nucleus (tiny!) Incoming α particles ~98% Pass straight through Small angle deflection ~2% Scattered ~1 in 8000 Reflected back! Rutherford's Famous Remark: "It was as if you fired a 15-inch shell at a piece of tissue paper, and it came back and hit you!" Key Insight Most of atom is EMPTY SPACE!

Summary of Observations

~98% of Particles

Observation: Passed straight through the gold foil with little or no deflection

Conclusion: Most of the atom is empty space

~2% of Particles

Observation: Scattered at various angles (small to large)

Conclusion: Came close to a positive charge concentration

~1 in 8000 Particles

Observation: Bounced straight back (backscattered)

Conclusion: Hit something tiny but massive and positive

Rutherford's Nuclear Model (1911)

Based on these surprising results, Rutherford discarded the Plum Pudding model and proposed a revolutionary new model of the atom:

Rutherford's Nuclear Model of the Atom (1911) + NUCLEUS Tiny but massive (+ charge) let electrons = [ { angle: 0, r: 200 }, { angle: Math.PI/3, r: 200 }, { angle: 2*Math.PI/3, r: 200 }, { angle: Math.PI, r: 200 }, { angle: 4*Math.PI/3, r: 200 }, { angle: 5*Math.PI/3, r: 200 }, { angle: Math.PI/4, r: 140 }, { angle: 3*Math.PI/4, r: 140 }, { angle: 5*Math.PI/4, r: 140 }, { angle: 7*Math.PI/4, r: 140 }, { angle: 0, r: 80 }, { angle: Math.PI, r: 80 } ]; for (let i = 0; i < electrons.length; i++) { let angle = electrons[i].angle + time; let x = 275 + electrons[i].r * Math.cos(angle); let y = 200 + electrons[i].r * Math.sin(angle) * 0.5; circle(x, y, 8, fill: #64b5f6); } Electrons orbit in shells The Size Scale If nucleus = marble (1cm) Atom = football stadium (~100m) Electrons = tiny specs on seats Key Features 1. Tiny, dense, + nucleus 2. Electrons orbit far away 3. Mostly empty space
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Key Features of the Nuclear Model

1. The Nucleus: A tiny, extremely dense, positively charged region at the center of the atom containing most of the atom's mass.

2. The Electrons: Negatively charged particles that orbit the nucleus at relatively large distances, like planets around the Sun.

3. Empty Space: The atom is mostly empty space—the distance between nucleus and electrons is enormous compared to their sizes.

4. Neutral Atom: For an atom to be neutral, the number of electrons must equal the number of protons.

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Explaining the Scattering Results

Why most particles passed through:

  • The atom is 99.999...% empty space
  • Alpha particles traveled through the empty regions without hitting anything

Why some particles scattered:

  • Came close to the positively charged nucleus
  • Experienced electrostatic repulsion (positive repels positive)
  • Closer approach = greater deflection

Why a few bounced back:

  • Direct collision with the tiny nucleus
  • Like a head-on collision with an immovable object

Interactive Gold Foil Simulation

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Alpha Particle Scattering Simulator

Objective: Understand how alpha particles interact with atoms based on different models. Watch how particles behave when they approach the nucleus at different distances.

Straight Through

0

Scattered

0

Reflected

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Click "Fire Alpha Particle" to send individual particles, or "Fire 10 Particles" to see statistical distribution.

Notice how particles passing far from the nucleus go straight through, while those passing close to or hitting the nucleus are deflected!

Chart: Scattering Distribution

Analysis: This chart shows the typical distribution of scattering angles observed in the Geiger-Marsden experiment. Notice how extremely rare large-angle scattering is—this corresponds to the tiny size of the nucleus!

Model Comparison: Before and After

Feature Thomson's Plum Pudding (1897) Rutherford's Nuclear Model (1911)
Positive Charge Diffuse sphere throughout atom Tiny, concentrated nucleus
Electron Location Embedded in positive sphere Orbiting nucleus at distance
Prediction for Alpha Scattering Most particles deflected slightly Most pass through, few scatter widely
Experimental Result ❌ Predictions failed completely ✅ Matched observations perfectly
Mass Distribution Distributed throughout atom Concentrated in nucleus

Worked Examples

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Example 1: Explaining Scattering Observations

Question: In the Geiger-Marsden experiment, why did most alpha particles pass straight through the gold foil without being deflected?

Solution:

  • According to Rutherford's model, atoms are mostly empty space
  • The gold foil is extremely thin (only ~400 atoms thick)
  • Alpha particles traveled through the empty regions between the nucleus and electrons
  • Since there was nothing to interact with, they continued in a straight line
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Example 2: Understanding Large-Angle Scattering

Question: Explain why some alpha particles were deflected at large angles (close to 180°) in the gold foil experiment.

Solution:

  • Alpha particles are positively charged (helium nuclei)
  • The gold nucleus is also positively charged
  • When an alpha particle passed very close to a gold nucleus, it experienced strong electrostatic repulsion
  • Particles approaching head-on or nearly head-on were deflected at large angles
  • The few particles that bounced straight back had collided almost directly with a nucleus
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Example 3: Why the Plum Pudding Model Failed

Question: If the Plum Pudding model were correct (positive charge spread throughout), what would the Geiger-Marsden results have looked like?

Solution:

  • In the Plum Pudding model, positive charge is diffuse and spread out
  • Alpha particles would experience many small deflections as they passed through
  • Very few particles would pass through undeflected
  • No particles would bounce straight back (there's no concentrated positive charge to stop them)
  • The actual results (~98% straight through, some backscattered) completely contradicted this model

Key Takeaways for CSEC

Essential Points for Your Examination

What You MUST Know

  • The setup of the Geiger-Marsden experiment
  • The three main observations and their percentages
  • How each observation led to a conclusion
  • The key features of Rutherford's nuclear model
  • Why the Plum Pudding model was rejected

Common Examination Questions

  • "Describe the Geiger-Marsden experiment"
  • "Explain the three observations and their significance"
  • "How did the results lead to the nuclear model?"
  • "Why did most alpha particles pass straight through?"
  • "Compare the Plum Pudding and Nuclear models"
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CSEC Examination Mastery Tip

Understanding the "Why" is Crucial: CSEC examiners don't just want you to memorize the results—they want you to explain why each result occurred based on the atomic model. When describing the experiment, always connect observations to conclusions.

  • Result: ~98% passed straight through → Conclusion: Atom is mostly empty space
  • Result: ~2% deflected → Conclusion: Positive charge concentrated in tiny region
  • Result: ~1 in 8000 reflected → Conclusion: Nucleus is tiny but massive

CSEC Practice Arena

Test Your Understanding

1
What percentage of alpha particles passed straight through the gold foil with little or no deflection?
~98%
~50%
~10%
~25%
Explanation: Approximately 98% of alpha particles passed straight through the gold foil. This surprising result indicated that atoms are mostly empty space, with nothing substantial in the path of most particles.
2
What did Rutherford conclude from the observation that some alpha particles were scattered at large angles?
Alpha particles were attracted by electrons
The atom had a uniform positive charge
The atom has a small, dense, positively charged nucleus
Electrons were located in the nucleus
Explanation: Large-angle scattering occurred when alpha particles (positive) came close to a concentrated positive charge. This led Rutherford to propose that the positive charge is concentrated in a tiny, dense region called the nucleus.
3
Which model of the atom was proven wrong by the Geiger-Marsden experiment?
Thomson's Plum Pudding model
Bohr's planetary model
Dalton's atomic theory
Rutherford's nuclear model
Explanation: The Geiger-Marsden experiment disproved Thomson's Plum Pudding model, which predicted that positive charge was spread diffusely throughout the atom. The actual results showed concentrated positive charge in a tiny nucleus.
4
Why did a very small number of alpha particles bounce straight back from the gold foil?
They were repelled by electrons
The gold foil was too thick
They collided almost directly with the tiny, massive, positive nucleus
They lost energy passing through the foil
Explanation: Particles that bounced straight back (backscattered) had collided almost head-on with the tiny, dense, positively charged nucleus. The strong electrostatic repulsion between two positive charges reversed their direction.
5
What is the relative charge and mass of an alpha particle?
Charge: -1, Mass: 1/1836
Charge: +1, Mass: 1
Charge: +2, Mass: 4
Charge: 0, Mass: 1
Explanation: Alpha particles are helium nuclei consisting of 2 protons and 2 neutrons. They have charge +2 (relative to electron charge) and mass approximately 4 atomic mass units.

Past Paper Questions

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CSEC Past Paper Practice

Question 1 (CSEC Physics 2021)

(a) Describe the Geiger-Marsden experiment and explain how it led to the proposal of the nuclear model of the atom.

(b) (i) In the experiment, most alpha particles passed straight through the gold foil with little or no deflection. What does this tell us about the structure of the atom?

(ii) A small number of alpha particles were scattered at large angles. Explain why this occurred.

Sample Answer:

(a) The Geiger-Marsden experiment involved firing alpha particles at a thin gold foil. A zinc sulfide screen detected the scattered particles. Most passed through, some deflected at angles, and a few bounced back. This led Rutherford to propose the nuclear model with a tiny, dense, positive nucleus.

(b) (i) This tells us that the atom is mostly empty space—alpha particles passed through the vast empty regions without encountering anything.

(ii) Large-angle scattering occurred when alpha particles came close to the positively charged nucleus and experienced electrostatic repulsion.

Question 2 (CSEC Physics 2020)

(a) State TWO differences between Thomson's Plum Pudding model and Rutherford's nuclear model of the atom.

(b) Explain why the Geiger-Marsden experiment results contradicted Thomson's model.

Sample Answer:

(a) Differences include:

  • Thomson: positive charge diffuse; Rutherford: positive charge concentrated in nucleus
  • Thomson: electrons embedded; Rutherford: electrons orbiting nucleus
  • Thomson: mass distributed; Rutherford: mass concentrated in nucleus

(b) Thomson's model predicted most particles would deflect slightly due to diffuse positive charge. The experiment showed most passed straight through and some reflected back—impossible with diffuse positive charge. This contradiction proved the nuclear model correct.

Question 3 (CSEC Physics 2019)

(a) What are alpha particles? Include their composition in your answer.

(b) In the Geiger-Marsden experiment, approximately 1 in 8000 alpha particles was scattered at angles greater than 90°. Explain this observation.

Sample Answer:

(a) Alpha particles are helium nuclei consisting of 2 protons and 2 neutrons. They have a positive charge of +2 and a mass of approximately 4 atomic mass units.

(b) Only particles that came extremely close to or collided directly with the tiny nucleus experienced the strong electrostatic repulsion needed to scatter at such large angles. Since the nucleus is so tiny, direct collisions are very rare (1 in 8000).

Summary: The Revolution in Atomic Physics

The Geiger-Marsden experiment represents one of the most important experiments in the history of physics. It completely transformed our understanding of matter at its most fundamental level. Before this experiment, atoms were thought to be diffuse blobs of positive charge with electrons scattered throughout. After, we knew that atoms have:

  • A tiny, dense, positive nucleus containing nearly all the atom's mass
  • Electrons orbiting far away at relatively vast distances
  • Almost entirely empty space between the nucleus and electrons

This discovery paved the way for Niels Bohr's improvements to the model and eventually led to our complete modern understanding of atomic structure and quantum mechanics.

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