Man's physical universe

xanabras

576 MAN IS MASTERINC; HIS MATERIAL WORLD

quite probable that radioactive sodium will soon be produced in

sufficiently large quantities to make it possible for use in replacing

radium for many purposes. This radioactive sodium has a half-life ' of

only fourteen hours, but it could be produced when needed. Radioactive

carbon has also been produced.

In 1937 E. Fermi showed that neutrons slowed to velocities of atoms

of hydrogen at ordinary temperatures are very effective in producing

radioactive substances.

In 1940 radioactivity was induced in uranium 235. In 1939 it was

shown that uranium atoms when bombarded with neutrons disintegrated

into nearly equal atomic fragments with the liberation of enormous

energies instead of inducing the emission of electrons, protons,

or helium nuclei as usually occurs.

In this splitting or fission, neutrons

are produced which can react with other uranium nuclei and thus continue

the process. In all previous experiments in which radioactive

substances had been produced, the radioactivity gradually decreased,

but here for the first time self-regenerating radioactivity had been

induced. At last a new source of power, that bound up with the atom,

has been tapped, although its practical application is still a dream. The

reason that uranium 235 may be a source of power is

that for each

disintegration about 200,000,000 electron-volts of energy are given out.

STUDY QUESTIONS

1. How do you account for the fact that electrons and alpha particles may be

shot through thin sheets of glass or metal containing thousands of layers of

atoms very close together?

2. Summarize the present thoughts concerning the nature of atomic nuclei.

3. Why is the periodic law best stated in terms of atomic numbers?

4. What is the atomic number of an element, and how is it obtained?

5. Compare atoms, neutrons, electrons, protons, positrons, deuterons, and alpha

particles as to mass, charge, and size.

6. Define isotope.

7. Why are the atomic weights of certain elements not whole numbers, while

the atomic weights of quite a number of elements are very nearly whole

numbers?

8. W'hat is deuterium, and how may it be obtained?

9. What is "heavy water"?

10. How is artificial radioactivity produced?

11. Summarize the evidence of the complexity of atoms.

12. State the periodic law, and show by one or more illustrations what it means.

13. What is meant by radioactivity?

14. Compare the radiations emitted by radioactive substances with the rayS

produced in discharge tubes.

15. What new ideas resulted from the discoveries of radioactivity?

'

Half-life is the time required for one half of any weight of a radioactive element to

disintegrate.

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