ATOMIC ENERGY — OLIPHANT 227 



present in a nucleus determines its positive electric charge and hence 

 the number of electrons which must rotate about it in order that the 

 atom, as a whole, may be electrically neutral. Thus the number of 

 protons determines what the atom is — if 1, the atom is hydrogen ; if 8, 

 oxygen ; if 92, uranium. The number of neutrons varies, so that there 

 may be several kinds of atoms of a given substance, called isotopes. 

 For example, hydrogen has 3 isotopes, the nuclei of which contain 1 

 proton with 0, 1, or 2 neutrons, while uranium has 2 principle isotopes 

 of mass 235 and 238 times the mass of elementary hydrogen, con- 

 sisting of 92 protons with 143 or 14G neutrons. 



The protons and neutrons in a nucleus may be altered in number 

 by bombarding with energetic charged particles, which can penetrate 

 inside against the repulsive forces due to the electric charge, or 

 neutrons may be added with greater ease since these do not experience 

 electrical repulsion. If the number of protons is changed, the atom 

 transforms into some other substance, and modern methods of al- 

 chemy, using cyclotrons and other accelerators to produce atomic 

 projectiles, enable us to change one substance into another at will, 

 though not yet in commercial quantities. 



Since the nuclear constituents are so tightly bound together, addi- 

 tion of particles to a nucleus leads, in general, to a release of energy 

 corresponding to this binding force. The energy released for every 

 atom undergoing a nuclear transformation is a million or more times 

 greater than the energy released in chemical combination. Thus, if 

 the nuclei of 4 atoms of hydrogen could be made to combine to produce 

 the nucleus of a helium atom, the energy released by 1 pound of hydro- 

 gen undergoing the reaction would be equivalent to 100 million kilo- 

 watt-hours, as compared with 3 or 4 kilowatt-hours produced by 

 burning 1 pound of coal. We shall return to this possibility later. 



The element uranium is the heaviest and most complex of the sub- 

 stances existing in the earth. Elements with more than 92 protons in 

 the nucleus are too unstable to have survived since the earth cooled 

 down about 3,000 million years ago. ^Y\\en a neutron is captured by 

 one of the two isotopes of uranium, the transformations which take 

 place differ from these occurring in other elements. The rarer isotope 

 of mass 235 undergoes a process that is called fission, splitting into 

 two large fragments which separate with great velocity, the energy 

 released being about 10 million kilowatt-hours for 1 pound of U-^" 

 undergoing fission. 



The U^^^ atom, after absorption of a neutron, splits into two atoms 

 of simpler structure and smaller mass, which separate with high veloc- 

 ity, their energy being dissipated as heat in the surrounding atoms 

 with which they collide. In addition, several neutrons are set free, 

 and if the surounding material is also U^^'*, these neutrons will be 

 absorbed and will produce several fresh fissions. Thus it is clear that 



