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.
UNIT VIII SECTION 4 MOLECULES ARE HELD TOGETHER BY ELECTRICAL OR MAGNETIC ATTRACTION OF ATOMS Introduction. Chemical changes represent rearrangements of atoms within molecules or the decomposition or combination of molecules to form smaller or larger molecules containing fewer or more atoms. What force holds atoms together within molecules? Are these forces the same for all atoms? By what means may these forces be overcome to bring about new arrangements of atoms within molecules? These are the questions which have puzzled chemists for many years. Some of them can now be answered, although the answers are very incomplete. Chemical Affinity Was First Used to Explain the Attraction between Atoms. In the thirteenth century, the Dominican monk, Alhertus Magnus, used the word "afifinitas," which expressed the idea that only atoms having a similarity or kinship would unite to form molecules. The Greek philosopher, Hippocrates, expressed the same idea when he maintained that "like unites only with like." In the eighteenth century Boerhaave and others arrived at the opposite conclusion, namely, that dissimilar atoms show the greatest tendency to combine. Before the knowledge of electricity developed, the nature of this force of affinity was not known. The terms "love" and "hate" were often used for want of anything better. It was known, however, that the affinity between the various atoms differed, and many attempts were made to arrange the elements in the order of their affinity for other elements. Today it is believed that chemical affinity is a force, electrical in nature, which acts between the different kinds of atoms to bring about chemical changes. The Properties of the Elements Depend upon Their Structures. Some metals are good conductors of electricity; others are not. Some metals may be pulled out to form fibers smaller in diameter than 577