Radioactivity & the Atom - Case Notes

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Author - Mary Kot/Caleb Arrington - Summer 1998

Background notes follow teaching notes.

DAY 1 Reading Assignment for Day 1: Assign each group the Badash article and then either Wynn or one of the two web sites.

• Badash, L (1966). How the "Newer Alchemy" Was Received. Scientific American 215: 88-95.
• Wynn, CM and AW Wiggins (1997). Physics’s Model of the Atom: Seeing the Unseeable. In The Five Biggest Ideas in Science. New York: John Wiley and Sons. pp 13-23.
• http://members.icanect.net/~zardoz/atom.htm (pages 1-8)
• http://www.yrbe.edu.on.ca/~mdhs/science/chemistry/ch2_2.htm

Beginning of Day 1 class period:

Quiz: 5-10 multiple choice questions over the reading material

Day 1 Discussion: Whole class discussion based on the following questions:

1. 1. What was the beginning of the atomic theory.
   Write these dates on the board: 384-322 B.C. and 460-370 B.C
2. How has the atomic theory changed from 1800 to 1900?
   Dates: 1803 (Dalton), 1873(Maxwell), 1895(Roentgen), 1895-1896(Becquerel), 1897 (JJ Thompson), 1900 (Planck)
3. How has the atomic theory changed from 1900-1910?
   Dates: 1902 (Rutherfoord and Soddy), 1905 (Einstein), 1909(Milikan), 1909 (Rutherford)
4. What made it possible for these revolutionary new, contradictory theories to be so easily accepted?
   Explanation in articles: Industrial revolution, Improved communication...
5. Who opposed the theory? Why?
6. How did the atomic theory change from 1911-1920?
   Dates: 1911 (Rutherford), 1912 (Bohr), 1919 (Rutherford)

DAY 2 Activity – coming soon - Dalton’s Law

DAY 3 Activity – coming soon - JJ Thompson and the electron

DAY 4: Reading Assignment: Making Indirect Measurements handout

WORD97, WORD95, MacWORD5.1, WordPerfect for Windows 5.x, Rich Text Format

Day 4 Activity: Making Indirect Measurements collect data

DAY 5 Complete calculations and discuss questions

DAY 6 Recreate Time line on board before class. Discussion Questions:

1. What can we say about theories in general?
2. How did Rutherford use experimental and theoretical approaches?
3. What predictive powers of the Rutherford theory did we discuss?
4. What metaphors have we discussed?
5. 5. Discuss the consensual nature of science in terms of the experiments on the atom.

BACKGROUND NOTES:

"It is not the nature of things for any one man to make a sudden violent discovery; science goes step by step, and every man depends on the work of his predecessors...Scientists are not dependent on the ideas of a single man, but the combined wisdom of thousands of men." Ernest Rutherford (1871-1937)

1803. Dalton proposes his theory of the atom

1873 James Clark Maxwell address to the British AAS the atom is incapable of growth, decay, generation or destruction

1895. penetrating X-rays discovered by W.C. Rontgen-discovered when cathode rays struck materials different type of ray emitted X ray
1896. Henri Becquerel discovered radiations emitting from a compound of uranium-most insisted that energy streaming from interior of atom could not reflect basic changes in atom itself. Might be: phosporescence, secondary radiation, selective absorption and re-radiation
1897. JJ Thompson discovered the electron Proposed a model for the structure of the atom-electrons have a negative charge so matter must have a positive charge. Metaphor: raisons stuck on the surface of a lump of pudding-Plum pudding model

1900 Max Planck vibrate atoms strong enough (heat an object until it glows) it emits radiation in separate bursts or quanta-quanta behave like particles or photons

1902. Ernest Rutherford and Frederick Soddy discover that radioactivity involves the sudden and spontaneous transformation of one atom into a chemically different atom-formulated the Rutherford-Soddy Law of radioactive decay. Rutherford and Soddy found that when a sample of a given type of radioactive substance was separated from its parent so that no new nuclei of the substance were being created then the number of the radioactive nuclei remaining as time elapsed decreased in a very special way. They also discovered that penetrating rays could be bent in a magnetic field: three types-alpha rays went to negative poll, beta rays to positive poll, gamma rays to neither. Radioactivity must arise from atom.

1909 Robert Milikan measured charge on single electron

1909 Showed alpha rays were helium atoms with both electrons removed. Thin walled glass . tube containing radon gas was sealed inside a thick walled glass tube containing mercury. The glass tube with mercury had a capillary tube attached to the top; capillary tube penetrated by two electrodes. Alpha rays emitted by gas passed from radon containing tube to mercury containing tube (alpha particles obtained electrons). Mercury level raised to force gas into capillary tube. A high voltage was applied to electrodes. Light produced observed with a spectroscope. Line spectrum identical to helium gas.

1911. Rutherford reveals the existence of the atomic atom as a minute concentration of mass and electric charge. Scattering experiments-high speed alpha particles from the radioactive element radon confined to a narrow beam by a hole in a lead block were made to randomly strike a very thin gold foil. Counted the number of particles scattered at different angles
1912. Scintillation method of detection. Alpha particle detector was a zinc sulphide screen. Each alpha particle striking the screen produces a tiny flash of green light (a scintillation) visible under a microscope.

Rutherford observed that almost all alpha particles went through the foil without appreciable deviation. All but one in 10, 000 alpha particles were deviated by less than 10 degrees.

• Conclusion: most of the alpha particles met only objects much less massive than themselves
• Conclusion: most of the gold atom as devoid of massive objects since most of the alpha particles passed through
• Conclusion: clearly on rare occasions the alpha particle encountered something much more massive than itself
• Assumption: an atom had a nucleus and that this minute massive nucleus (smaller than 10^–12 cm in diameter) was electrically charged and contained nearly half all the atoms mass
• Theoretical: calculated what fraction of alpha particles would be deflected by the electrical push exerted by the nucleus through angles greater than 10, 20 ..., etc degrees
• Prediction: The positive charge and mass of every atom are confined to a particle smaller than 10 ^–12 cm in diameter
• Detailed numerical agreement between these calculations and the experimental results amassed by assistants Geiger and Marsden
• 1919 Repeated experiments with different films of light and heavy elements like copper and silver showed the relative number of wide angle deflections increases with atomic number
• 1919 Rutherford discovers protons. Rutherford model atoms consist of a positively charged nucleus surrounded by a system of electrons kept together by attractive forces from the nucleus total negative charge of electrons is equal to the positive charge of the nucleus. Nucleus is assumed to be the seat of the essential part of the mass of the atom Problem with theory-electrons should spiral into the nucleus and give off color as they do

1912. Neils Bohr later combined the quantum theory of light to form the basis of theory of the atom-onion metaphor

• Electrons do not spin into the nucleus
• Rules fit observation regardless how they conflict with theories of the time
• Rules:
  • Electrons orbit at certain distances form the nucleus
  • Atoms radiate energy when electrons jump from higher energy orbit to lower orbit