THE GROWTH OF CHEMICAL IDEAS 143 



If too much of the common isotope of uranium, the 238 

 isotope, is present, the neutrons will be absorbed by the atoms 

 of 238 and will not be available to disintegrate the 235. 



The production of the atomic bombs that were dropped 

 on Japan depended on the working out of these problems on 

 an engineering scale. The uranium 235 was separated from 

 ordinary uranium by very laborious processes that produced 

 only a very small amount in each piece of apparatus, but by 

 building enormously large factories enough of the isotope 

 could be obtained for effective use in bombs. At the same 

 time, a new element, plutonium, was produced, this ma- 

 terial being made by the exposure of uranium 238 to neu- 

 trons supplied from uranium 235, the whole reaction taking 

 place in a structure called a pile. 



Plutonium was first made in a cyclotron. A neutron adds 

 uranium to an atom of 238 to produce an unstable uranium 

 isotope, which emits an electron from its nucleus and turns 

 into a new element, number 93; and this in its turn emits 

 an electron and turns into plutonium, el-ement 94. Plu- 

 tonium is similar in its radioactive properties to uranium 

 235. Chemically, of course, it differs from uranium and 

 can be separated from it by chemical means. Plutonium in 

 sufficient quantity undergoes a self-prop'a gating fission like 

 uranium 235, so that atomic bombs can be made either by 

 the use of the uranium isotope 235 or by the use of plutonium 

 produced from uranium in a pile. 



