RADIUM, ATOMIC ENERGY, AND ATOMIC STRUCTURE 549 



are inversely proportional to the wave-lengths of corresponding 

 lines in their X-ray spectra. These atomic numbers have been 

 determined for most of the elements, the atomic weights of which 

 lie between those of aluminium and uranium. In the following 

 table, the atomic numbers for these elements are given and, for 

 the sake of greater completeness, numbers for the twelve elements 

 preceding Al have been inserted also. 



ATOMIC NUMBERS (MOSELEY) 



* The atomic numbers 59-72 are those of the metals of the rare earths: Pr 59, Nd 60, 61, 

 Sa 62, Eu 63, Gd 64, Tb 65, Dy 66, Ho 67, Er 68, Tm 69, Yb 70, Lu 71, - 72. 



It will be seen that there is a whole number available for every 

 known element, up to and including uranium, and not omitting 

 the rare earths which have no satisfactory place in the periodic 

 system. There are four blank numbers in the table, which cor- 

 respond to three spaces below Mn in the periodic system and one 

 under gold, and two more amongst the rare earths, indicating 

 only six elements with atomic weights less than that of uranium 

 yet to be discovered. The atomic numbers of argon and potas- 

 sium place them in the chemically correct order, while the atomic 

 weights do not. The same is true of cobalt and nickel and of 

 tellurium and iodine. 



The atomic numbers represent the number of free positive 

 charges of electricity in the nucleus of the atom of each element. 

 It must be noted that the nucleus also contains, in all cases except 

 hydrogen, a number of bound positive charges associated with an 



