PRINCIPLES OF NAVAL ENGINEERING 



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A COMMON HYDROGEN 



B DEUTERIUM 



C TRITIUM 



Figure 24-3.— Isotopes of hydrogen. 



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the symbol of the element is the atomic mass 



number; thus the superscript indicates which 



isotope of the element is being referred to. 



The geneal symbol for any atom is thus 



Z 

 X 

 A 



where 



X symbol of the element 

 Z atomic number (number of protons) 

 A atomic mass number (sum of the number 

 of protons and the number of neutrons) 



Of the known 103 elements, there are ap- 

 proximately 1000 isotopes, most of which are 

 radioactive. 2 Figure 24-4 gives the nuclear 

 compositon of various isotopes. 



RADIOACTIVITY 



All isotopes with atomic number Z greater 

 than 83 are naturally radioactive and many more 

 isotopes can be made artifically radioactive by 

 bombarding with neutrons which upset the 

 neutron-proton ratio of the normally stable 

 nucleus. 



Naturally radioactive isotopes undergo ra- 

 dioactive decomposition , thereby forming lighter 

 and more stable nuclei. Radioactive decom- 

 position occurs through the emission of an 



^For a detailed discussion of nuclear stability, see 

 Francis W. Sears and Mark W. Zemansky, University 

 Physios (3d ed.; Reading, Mass.: Addison-Wesley 

 Publishing Company, Inc., 1964), p. 997. 



alpha particle or a beta particle . One or 

 more gamma rays may also be emitted with 

 the alpha or beta particle. 



An alpha particle (symbol a) is composed 

 of two protons and two neutrons. It is the 

 nucleus of a helium (2He^) atom, has an elec- 

 trical charge of +2, and is very stable. In 

 the decay process to a more stable element, 



Figure 24-4.— Nuclear composition 

 of various isotopes. 



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