ATOMIC WEIGHTS ASTON. 193 



This enables the most sweeping simplifications to be made in our 

 ideas of mass. The original hypothesis of Prout, put forward in 

 1815, that all atoms were themselves built of atoms of protyle, a 

 hypothetical element which he tried to identify with hydrogen, is 

 now reestablished, with the modification that the primordial atoms 

 are of two kinds : Protons and electrons, the atoms of positive and 

 negative electricity. 



The Rutherford atom, whether we take Bohr's or Langmuir's 

 development of it, consists essentially of a positively charged central 

 nucleus around which are set planetary electrons at distances great 

 compared with the dimensions of the nucleus itself. 



As has been stated, the chemical properties of an element depend 

 solely on its atomic number, which is the charge on its nucleus ex- 

 pressed in terms of the unit charge, e. A neutral atom of an element 

 of atomic number N has a nucleus consisting of K-\-N protons and 

 K electrons, and around this nucleus are set N electrons. The weight 

 of an electron on the scale we are using is 0.0005, so that it may be 

 neglected. The weight of this atom will therefore be K-\-N, so that 

 if no restrictions are placed on the value of K any number of isotopes 

 are possible. 



A statistical study of the results given above shows that the 

 natural restrictions can be stated in the form of rules as follows : 



In the nucleus of an atom there is never less than one electron to 

 every two protons. — There is no known exception to this law. It is 

 the expression of the fact that if an element has an atomic number 

 N the atomic weight of its lightest isotope can not be less than 2N. 

 Worded as above, the ambiguity in the case of hydrogen is avoided. 

 True atomic weights corresponding exactly to 27V are known in the 

 majority of the lighter elements up to A 36 Among the heavier ele- 

 ments the difference between the weight of the lightest isotope and 

 the value 2N tends to increase with the atomic weight; in the cases 

 of mercury it amounts to 37 units. The corresponding divergence of 

 the mean atomic weights from the value 2N has, of course, been 

 noticed from the beginning of the idea of atomic number. 



The number of isotopes of an element and their range of atomic 

 weight appear to have definite limits. — Since the atomic number only 

 depends on the net positive charge in the nucleus there is no arith- 

 metical reason why an element should not have any number of iso- 

 topes. So far the largest number determined with certainty is 6 in 

 the case of krypton. It is possible that xenon has even more, but 

 the majority of complex elements have only two each. The maximum 

 difference between the lightest and heaviest isotope of the same ele- 

 ment so far determined is 8 units in the cases of krypton and xenon. 

 The greatest proportional difference, calculated on the lighter weight. 



