Lesson 2 Matter

  Rutherford's Atomic Theory

© Wikimedia Commons
B2.15 Ernest Rutherford

In 1911, physicist Ernest Rutherford performed an experiment to test Thomson’s plum pudding model. In the experiment, Rutherford shot very small alpha particles (which are positively charged helium ions) at a thin sheet of gold foil. Rutherford expected all of the particles to be deflected just a bit as they passed through the “plum pudding.” He found that most of the alpha particles he shot at the foil were not deflected at all. They passed through the foil and emerged undisturbed. Occasionally, however, particles were scattered at huge angles and a few of them even bounced right back.

  Did You Know?

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B2.20 Geiger counter

Rutherford worked with Professor Hans Geiger in creating the Geiger counter. A Geiger counter is a devise that is used to detect radiation, which can have serious health effects. Click on the photo to listen to the detection of radiation.

  Watch This

Rutherford's Alpha Scattering Experiment ©Blausen

Watch this video that will provide an overview of Rutherford’s experiment and how he interpreted the results.

  Rutherford's Atomic Theory Continued

© Wikimedia Commons
B2.16 Rutherford’s gold foil experiment

To understand how surprising these results were, imagine shooting a rifle at a mound of loose snow. You would expect most bullets to emerge from the opposite side. But imagine your surprise if a bullet reflected back at you! If that happened, you might guess that there was a brick of hard material inside the snow mound.

These results lead Rutherford to propose that most of an atom was empty space but with a positively charged center that contained most it's mass—Rutherford had discovered protons and the nucleus (from the Latin for “little nut”). Rutherford also proposed that the nucleus contained a neutral particle, which was eventually named “the neutron.” But it was not until 1932, that James Chadwick was able to prove that these neutral particles exist.

© Wikimedia Commons
B2.17 Expected pathway of alpha particles in the Thomson model and the Rutherford model of the atom

  Virtual Lab

Rutherford Experiment © The Concord Consortium

• Background Information
  
• Procedure
• Analysis

Work through this simulation to help visualize how changing the placement of the positive charges will affect the deflection of alpha particles.

Click on the procedures tab to continue.

1. Click on the play icon to open the virtual lab. The lab can also be found at https://quick.adlc.ca/rutherford
2. Uncheck “Show field generated by positive charges.”
3. Move the cursor for “Set Spread of Positive Charge” to maximum diffuse.
4. Click “Shoot alpha particles.”
5. Observe the pathways and interactions of the alpha particles with the positive charges.
6. Run the simulation for 20 s. Then click “Stop.”
7. Take a screen shot of the results.
8. Click “Reset.”
9. Move the cursor for “Set Spread of Positive Charge” to maximum concentrated.
10. Click “Shoot alpha particles.”
11. Observe the pathways and interactions of the alpha particles with the positive charges.
12. Run the simulation for 20 s. Then click “Stop.”
13. Take a screen shot of the results.
14. Click on the analysis tab to complete the analysis questions.
    

1. What pattern, similar to the Thomson model of the atom, did the alpha particles make when the spread of positive charge was set to diffuse?
   
   
   Check your work.
   

   Most were slightly deflected.
   
   

   
   

   

   © Concord Consortium
   B2.18 electron pathway through the atom

   
   

   
   
2. What pattern, similar to the Rutherford model of the atom, did the alpha particles make when the spread of positive charge was set to concentrated?
   
   
   Check your work.
   

   Some were deflected at great angles.
   
   

   
   

   

   ©Concord Consortium
   B2.19 electron pathway showing electrons colliding with the nucleus
   

   
   

   
   

Many years later, reflecting on his reaction to these results, Rutherford said, "It was quite the most incredible event that has ever happened to me in my life. 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."

By recording the number of the alpha particles deflected at large angles, Rutherford was able to estimate the size of the nucleus. According to his calculations, the radius of the nucleus is at least 10,000 times smaller than the radius of the atom!

© Wikimedia Commons
B2.21 Model of Rutherford’s atom

An analogy that can be used to help you visualize Rutherford’s model of the atom is a planetary model (sometimes called a nuclear model), which puts all of the protons in the nucleus and the electrons orbiting around the nucleus.

  Digging Deeper

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© Wikimedia Commons
B2.22 nuclear powerplant

The idea that the nucleus of an atom is made up of smaller particles also led to the first splitting of an atom (nuclear fission). Two students of Rutherford, Ernest Walton and J.D. Cockcroft, successfully split a lithium ion in 1932. This was a crucial first step in the development of nuclear power.

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  Read This

  Practice Questions

1. Describe the evidence that Rutherford collected in the gold foil experiment.
   
   
   Check your work.
   

   He shot alpha particles at thin gold foil to watch for any pathway changes of the alpha particles as they passed through the gold foil. He was expecting most to pass through with slight deflection. In reality, most passed through without being deflected, with a few being deflected substantially.
2. Describe how Rutherford interpreted the evidence he collected in the gold foil experiment.
   
   
   Check your work.
   

   There were two key interpretations.
   
   

   1. Since most alpha particles were deflected, atoms are mostly empty space.
      
      
   2. Since some alpha particles were deflected at huge angles, the interior of the atom must contain a central core that is very dense and positively charged.
3. Describe how Rutherford modified Thomson’s model of the atom in light of the evidence gathered during the gold foil experiment.
   
   
   Check your work.
   

   There had to be a central core that contained almost all of the mass and the positive charges, instead of it being spread evenly about the atom. The atom had to be composed of mostly empty space, with electrons circling around it.
4. How would you describe Rutherford’s model of the atom? What analogy would you use for Rutherford’s model of the atom? Use your own words!
   
   
   Check your work.