March 3, 1922] 



SCIENCE 



225 



placed on the value of K any number of iso- 

 topes is possible. 



The first restriction is that excepting in the 

 case of hydrogen K can never be less than N 

 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 lim- 

 ited, for so far no two isotopes of the same 

 element have been found differing by more 

 than 10 per cent, of its mean atomic weight, 

 the greatest difference is eight units in the 

 case of krypton. The actual occurrence of 

 isotopes does not seem to follow any law at 

 present obvious, though their number is prob- 

 ably limited by some condition of stability. 



Protons and electrons may therefore be re- 

 garded as the bricks out of which atoms have 

 been constructed. An atom of atomic weight 

 m is turned into one of atomic weight m -f- 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 of these new configurations are pos- 

 sible they will represent elements of the same 

 atomic weight but with different chemical 

 properties. Such elements we have pointed 

 out above are called isobares, and are already 

 known to exist among the radioactive elements. 

 (See Table II). 



The element hydrogen, it will be noted, is 

 unique in that its nucleus weight, 1.008, ex- 

 hibits a departure from the rule of integers 

 followed by the isotopes of all the other 

 elements investigated. It will be noted, how- 

 ever, that it is the only atom in which the 

 nucleus is not composed of protons and elec- 

 trons closely packed together. It can be shown 

 that with close packing of protons and elec- 

 trons there must follow a reduction in effective 

 mass, and that when four protons and two 

 electrons are closely packed togetlier as they 

 must be in alpha particles, the nuclei of helium 

 atoms, the resultant effective mass must be 

 somewhat less than four times that of the 

 hydrogen nucleus. 



VI. The Dimensions op Atomic Nuclei, 

 Their Electric Charges and 

 Fields of Force 

 While phenomena connected with the scat- 

 tering of a rays have led to such profound mod- 

 ifications in our views of atomic structure, it is 

 of interest to note that through the agency oE 

 these same a, rays we are likely to make still 

 further advances in the problem of determin- 

 ing the ultimate structure of matter. Through 

 the attacks now being rigorously pi'essed by 

 Eutherford and his associates, the structure of 

 the nuclei of atoms is slowly but steadily being 

 revealed. Through the bombardment of atomic 

 nuclei by a rays it has been found that the 

 electric charges on atomic nuclei can be meas- 

 ured with a high degree of precision, estimates 

 of the diameters of nuclei can be made, the 

 field of electric force about a nucleus can be 

 examined^ and the structure of the nucleus 

 itself can be broken down. 



(a) Nuclear charges. 



In his early experiments, Eutherford had 

 shown from the experiments of Geiger and 

 Marsden on the scattering of a rays that the 

 charge on the nuclei of atoms of gold was 

 within 20 per cent, equal to 100 e. More 

 recently Chadwick-" has shown by the use of 

 direct and more refined methods that the 

 charges on the nuclei of three types of atoms, 

 namely, those of platinum, silver and copper, 

 have the value of 77.4e, 46.3e and 39.3e respec- 

 tively. As the atomic numbers of these ele- 

 ments are 78, 47 and 39, it will be seen that 

 these results strongly confirm the view put 

 forward by Eutherford as a result of the 

 experiments of Moseley and others, which indi- 

 cate that the nuclear charge is equal to Ne, N 

 being the atomic number of the element. 



(b) Nuclear dimensions and nuclear electric 

 fields of force. 



As mentioned above, Rutherford has shown 

 by experiments on the scattering of a rays that 

 the dimension-s of atomic nuclei must be ex- 

 ceedingly small. For example, wlien high 

 speed a particles collided with atoms of gold 

 they were found to be turned back in their 

 path at a distance of 3 X 10—13 em. between 



20 Cliadwick, Phil. Mag., Dec, 1920, p. 734. 



