1921] on Isotopes and Atomic Weights 309 



nucleus around which are set planetary electrons at distances which 

 are 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 expressed 

 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 ■ 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. 



The first restriction is that, excepting in the case of hydrogen, 

 K can never be less than X, for the atomic weight of an element is 

 always found to be equal to, or greater than, twice its atomic number. 

 The upper values of K also seem to be limited, for, so far, no two 

 isotopes of the same element have been found differing by more 

 than 10 per cent of its mean atomic w T eight ; the greatest numerical 

 difference is eight units in the case of krypton. The actual occur- 

 rence of isotopes does not seem to follow any law at present obvious, 

 though their number is probably limited by some condition of 

 stability. 



Protons and electrons may therefore be regarded as the bricks 

 out of which atoms have been constructed. An atom of atomic 

 weight m is turned into one of atomic weight m + 1 by the addition 

 of a proton plus an electron. If both enter the nucleus the new 

 element will be an isotope of the old one, for the nuclear charge has 

 not been altered. On the other hand, if the proton alone enters the 

 nucleus, and the electron remains outside, an element of next higher 

 atomic number will be formed. If both these new configurations 

 are possible they will represent elements of the same atomic weight, 

 but with different chemical properties. Such elements are called 

 " isobares," and are actually known among the radioactive elements. 



The case of the element hydrogen is unique, for its atom appears 

 to consist of a single proton as nucleus with one planetary electron. 

 It is the only atom in which the nucleus is not composed of a number 

 of protons and electrons packed exceedingly close together. Theory 

 indieates that when such close packing takes place the effective mass 

 will be reduced, so that when four protons are packed together with 

 two electrons to form the helium nucleus, they will have a weight 

 rather less than four times that of the hydrogen nucleus, which is 

 actually the case. 



It is not to be supposed that the whole-number rule is of exact 

 mathematical accuracy, for the unit of the oxygen scale is a "packed" 

 proton + an electron, and its value will certainly alter slightly with 

 the degree of packir.g. On this account, it is of the greatest impor- 

 tance to push the accuracy of methods of atomic weighing as far as 

 possible, for variations from the whole-number rule, if they could be 



