ATOMIC ENERGY — JOHNS 181 



erties. Heavy water is built up from heavy hydrogen and oxygen. 

 It is very expensive and is used by the ton in the atomic-energy plant 

 at Chalk River. Natural carbon consists of 99 percent of eC^^ and 1 

 percent of 6C^^ the former having as nucleus 6 protons and 6 neutrons 

 and the latter 6 protons and 7 neutrons. The respective atomic weights 

 are 12 and 13. Both are carbon with atomic number 6. Uranium as 

 found in nature consists of three isotopes, a trace of U-^* with 142 

 neutrons, 0.7 percent of U^^^ with 143 neutrons, and 99.3 percent of 

 ■[J238 -^[lY^ i4g neutrons. It turns out that the valuable one for securing 

 the release of atomic energy is U"®, but it is found mixed with 139 

 parts of U-^^. If the proportion in nature had been reversed, the 

 Germans would have won the war. 



ATOMIC ENERGY RELEASED BY NATURE 



We have seen that chemical actions, which are always concerned 

 with the superstructure of the atom, yield comparatively little en- 

 ergy. To secure larger amounts the nucleus of the atom must be in- 

 vaded. I shall cite two illustrations of such energy release which is 

 going on in nature. 



The first is radioactivity. It has been known for about 50 years 

 that the element radium is continuously shooting out projectiles at 

 terrific speeds. Such emanations are of three types, a-particles which 

 are the nuclei of helium atoms, y8-particles or electrons, and y-rays 

 which are similar to X-rays. By a series of transformations an atom 

 of radium, ssRa^^^, with mass number 226 and atomic number 88 gives 

 off, besides y-rays, five a-particles and four y8-particles to become an 

 atom of lead, 82Pb=^°% with mass number 206 and atomic number 82. 

 The mass numbers check, since each helium atom has a mass number of 

 4 and 5X4=20 is the loss in mass number. The atomic numbers 

 also check, since 5X2=10 units of positive charge are lost with the 

 five a-particles, and four unit negative charges with the four elec- 

 trons — a net loss of six units of positive charge from the nucleus. 

 Radium is being transformed into lead before our eyes. In about 

 1,600 years half our radium will be so transformed. In another 1,600 

 years half of what remained, and so on. Hence we speak of the half- 

 life of radium as 1,600 5^ears. Always some radium will remain. 



Our second illustration is of special interest to the astronomer. 

 We are told that the sun in every second of time is giving out 10-° 

 kilowatt hours of energy and has been doing this for some 10° or 

 10^° years. Using Einstein's equation E=^mc\ we find that this is 

 equivalent to the transformation of 250 million tons of matter into 

 energy every minute over this tremendous span of time. Professor 



