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Natural Science - Year II

Unit 54: Rutherford and Atomic Structure

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History Weblecture for Unit 54

This Unit's Homework Page History Lecture Science Lecture Lab Parents' Notes

History Lecture for Unit 54: Rutherford and Atomic Structure

For Class


The Empty Atom

We've already met Joseph John Thomson -- he isolated electrons in a beam and determined their charge/mass ratio meant they had a mass smaller than the total mass of the smallest known atom (hydrogen). This information gave independent confirmation that the Curies were right: atoms were made of sub-particles that could be separated from one another. This was a radical departure from the solid, unbreakable atom proposed by John Dalton, and many of the physicists between 1890 and 1920 had a difficult time accepting the drastic changes in theories of matter.

The Plum Pudding Model

Thomson used this information and that of Lord Kelven's theories about the radiation patterns of polonium and radium to propose a "plum pudding" concept of the atom. In his theory, the electrons would be embedded in a cloud of positive charge, moving constantly to stay in equilibrium. Read through the description of the Kelvin-Thomson model.

  • What are Coulomb forces?
  • What would an atom to disintegrate?
  • What problems did the theory have in explaining large atoms with many charged particles?

Rutherford's Golden Experiment

Ernest Rutherford was a student of J. J. Thomson at the Cavendish Laboratory in Cambridge. After graduating with his degree, he spent a decade teaching in Montreal, Canada, where he also did extensive research into alpha and beta radiation, discovering that each radioactive element had a unique constant associated with the rate at which its unstable atoms decayed. He defined the concept of half-life as the time it takes one-half of the unstable atoms in a sample to decay.

Around 1907, Rutherford returned to England to work at the University of Manchester with Hans Geiger (of Geiger counter fame). His experiments at Manchester led him to propose a new model for the atom, called the "planetary" model, in which electrons orbited at a great distance a small concentrated nucleus of positive charge and high mass.

Read through the sections on Alpha Scattering, Geiger and Marsden, and "Shells off Tissue Paper" starting here and using the links on the left. Pay attention to the little graphics below the list of topics.

Then try out this browser-based Scattering Simulation .

  • Push the "run" button to send alpha particles through the atoms.
  • In the alpha particle box, turn on the "traces" option.
  • What happens if you increase alpha particle energy to maximum?
  • What happens if you decrease alpha particle energy to minimum?
  • Push the nucleus button (ball of red and white spheres) to watch the interactions on a nuclear scale. Do any alpha particles actually hit the nucleus?
  • What happens if you increase the number of protons and neutrons to maximum? what happens if you decrease them to the minimum allowed?
  • Use the button at the bottom of the page to switch to the Plum Pudding Atom. Rerun your "experiment". What should happen in the plum pudding model? What is different from this prediction in Rutherford's actual experiment?

How does the location where the ricocheting alpha particle strikes the screen vary with gun location and initial energy?

  • What is an alpha particle?
  • Why is an alpha particle deflected from the nucleus even if it doesn't actually hit it?
  • Why does a larger aberration indicate a small, concentrated nucleus?
  • How big was the nucleus of Rutherford's gold atom?
  • What problems remained in Rutherford's theory?

Bohr's Atom

Niels Bohr also studied physics under J. J. Thomson at the Cavendish laboratory, and then was a student of Rutherford's in Manchester. HIs time in England allowed him to become acquainted with most of the top British scientists. When he taught at Copenhagen University, he drew students from all of Scandinavia and Germany, including Werner Heisenberg. Bohr's major contribution to our understanding of the structure of the atom was the realization that electrons can only exist in specific orbitals, balanced between their own kinetic energy, the attraction of the positive nucleus, and the repulsion of other electrons might also surround the nucleus.

Read about Bohr's life in the Nobel Prize biography collection.

Then use the Model of Bohr's atom to see how photon energy will cause electrons in the hydrogen atom to change orbits. [Requires FLASH.]

  • The electron starts in a ground state. Click on the boxes at the bottom of the simulation screen to launch a photon at the electron. Which photons cause the electron to change orbits?
  • What happens if you fire a photon at electrons in a high orbit?
  • What happens if you excite an electron and then wait?
  • What happens if you set the photon value to some random number and fire the photon?

Study the Standing Waves in Bohr's atomic model site. Watch the short video preview, then see if you can use the Geogebra simulation to match an electron "wave" to the orbit so that it doesn't move.

Study/Discussion Questions:

Further Study/On Your Own