LEARNING AIMS
After studying this unit you should be able to:
1.
use the key vocabulary connected with the nuclear atom;
2.
discuss the history of atom-related discoveries;
3.
draw a simple model of an atom;
4.
write notes.
KEY WORDS
Atom, constituent, electron, emit, ionize, neutron, nucleus, nuclide, particle, proton, radiation, scintillation
LEAD IN
EX 1
Draw a simple model of an atom. Show it to your partner and compare your models.
EX 2
Show your models in the class and take a vote whose model is the most precise. (To compare, look at picture no. 1 on page 5)
EX 3
Read the first paragraph of the text.
Thomson’s “plum-pudding“ model of the atom
Atoms are not charged. Since electrons are negatively charged constituents of an atom, the rest of the atom must be positively charged. Thomson’s model of the atom shown below consists of a positively charged “pudding“, with negatively charged “plums“ or “raisins“ (electrons) embedded in it.
SPEAKING
EX 4
What is the difference between the model described in the paragraph and your model?
Discuss in pairs and together draw this model. Then check picture no. 2 on page 5 to see how close you were.
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LISTENING
EX 1 a.
Listen to the whole text and take notes. b.
Compare your notes with your partner and complete each other’s missing information.
c.
Listen again to check the correctness of your notes.
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EX 2
Now listen again and complete the missing words.
Thomson’s “plum-pudding” model of the atom
1________________ are not charged. Since electrons are negatively 2________________ constituents of an atom, the rest of the atom must be positively charged. Thomson’s model of the atom shown below consists of a positively charged “pudding“, with negatively charged “plums“ or “raisins“ (electrons) embedded in it.
Discovery of the 3________________
In 1899, Ernest Rutherford discriminated between three types of 4________________ radiation emitted by unstable nuclides: alpha, beta and gamma radiation. In 1909, researchers in his group were using alpha particles to probe gold atoms. Geiger and Marsden fired
5________________ 6 ________________ at a thin gold foil. They used a zinc sulphide screen (a simple scintillation detector) to see where the alpha particles ended up.
Geiger and Marsden discovered that:
Most of the alpha particles went straight through the gold foil without any change in direction and without any loss of
7 ________________.
Some alpha particles were deflected and some even rebounded.
Rutherford set about interpreting the data. The most obvious conclusions to be drawn from these experiments were: most of an atom is empty space the positive charge is concentrated in the nucleus the mass is concentrated in the nucleus
Rutherford was able to determine the size of the nucleus by working out the force needed to give the necessary 8________________ deflections, using Coulomb’s law (an 9________________ -________________ law for the force between two charges). He found that while an atom’s diameter might be around 10 -10 m, the diameter of the nucleus is typically around 10 -15 m – 100 000 times smaller!
Rutherford’s new model posed serious problems that classical physics could not answer. The electrons had to be in orbit, since
10________________ 11________________ would be captured by the nucleus, but
12________________ electrons are being constantly accelerated according to classical physics, accelerated charges should emit radiation (however, they do not).
The evidence for the nuclear structure of the atom was so strong that it was classical physics that had to give way. Niels Bohr used atomic line spectra to show that 13________________ 14________________ had specific energy levels and Louis de Broglie provided a theoretical mechanism for these states by linking particle and wave properties.
The nuclear atom
Moseley’s suggestion that positive charge should be quantized and Chadwick’s discovery of the 15________________ completed the basic picture. The approximate 16________________ of an atom = 10 -10 m, and the approximate radius of the nucleus =
17________________.
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EXTRA PRACTICE
Describe the reaction in the picture.
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LEAD IN
EX 2
Picture 1 - Model of an atom
SPEAKING
EX 4
Picture 2 - Thomson’s “plum-pudding” model of an atom
KEY
LISTENING
EX 2
1. atoms, 2. charged, 3. nucleus, 4. ionizing, 5. alpha, 6. particles, 7. energy, 8. alpha, 9. inverse-square, 10. static,
11. electrons, 12. orbiting, 13. electron, 14. orbits, 15. neutron, 16. radius, 17. 10 -15 m.
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VOCABULARY - UNIT 5 plum-pudding approximate atomic line spectra capture charged constituent deflect deflection determine diameter discovery discriminate embed emit evidence foil ionize nucleus nuclide orbit pose probe radiation radius rebound scintillation screen unstable wave
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“hrozinkový” pudink přibližný čárové atomové spectrum zachytit nabitý součást, složka (základní) odklonit, odchýlit odklon, odchýlení určit průměr objev rozlišovat zapustit, vložit vyzařovat důkaz fólie ionizovat jádro nuklid orbit, oběžná dráha představovat zkoumat záření poloměr, radius odrazit třpyt, jiskření stínítko nestálý vlna
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