DESTRUCTIVE AND NON-DESTRUCTIVE METHODS 183 



According to the present view, the nucleus of an atom is formed from 

 protons of mass 1 and charge + 1 and neutrons of mass 1 and charge 0. 

 The atomic number corresponds to the number of protons and all the 

 atoms of a given element have the same number of protons in the nucleus. 

 But the total number of protons and neutrons can vary. To designate the 

 different isotopes of an element, its symbol is written preceded by a number 

 corresponding to the atomic number (i.e. to the number of protons) and 

 it is followed by an index corresponding to the number of nucleons 

 (protons 4- neutrons). 



The three isotopes of hydrogen, protium, deuterium and tritium are 

 written as follows: 



iHi iH2 iH3 



and the four most important isotopes of carbon thus : 



pll pl2 pi3 r>14 



6^ 6^ 6^^ e^-' 



Since the atomic number is the same for the several isotopes of an ele- 

 ment, it is often omitted, and only the number of nucleons is noted. 

 Certain isotopes are stable and occur naturally and they may be concen- 

 trated from these natural sources. The most important as far as the 

 biochemist is concerned, are lH^ qC^^, vN^^ and gO^*. 



It is also possible to prepare artificial isotopes by bombardment with 

 protons, neutrons, a-particles, etc. These isotopes are radioactive and after 

 varying lengths of time they undergo transmutation accompanied by the 

 emission of electrons. The determination of stable isotopes is carried out 

 with the mass spectrometer and radioactive isotopes are determined by 

 measuring their degree of radioactivity. 



If we consider a homogeneous population of radioactive atoms, a con- 

 stant proportion of this population will decay in any given period of time. 



An interesting characteristic of each isotope is its half life, a figure 

 corresponding to the time required for exactly half of the total number of 

 atoms to decay. It is equal to infinity for the stable isotopes. It is twelve 

 years for tritium, 5900 years for eC^^, but only fifteen hours for iiNa24 aj^(i 

 20*5 minutes for qC^^. 



One of the most useful applications of isotopes is in the study of meta- 

 bolic problems and the determination of a precursor -product sequence. 

 Suppose that we wish to know whether A is converted to B inside the 

 organism. We synthesize A; introducing atoms of an isotope, we administer 

 it to the organism and then we isolate the compound B after some little 

 time. We degrade this compound and determine the distribution of the 

 isotopic atoms in the molecule. This procedure often casts useful light on 

 the mechanism of the conversion. 



Isotopes also enable us to measure the speed of synthesis or breakdown 



