218 ANNUAL RErORT SMITHSONIAN INSTITUTION, 19^7 



re-radiated by each electron, and then set his pupils to measure just 

 what this fraction was in specific cases. From the experimental 

 results he was thus able to calculate the number of electrons which 

 performed the re-radiation in each case. These results indicated 

 that the number of such acting electrons in each atom was about half 

 the value of the chemical atomic weight of the atom. Thus first were 

 counted the electrons in an atom. 



Rutherford and his pujiils, aided by the mathematical analysis of 

 Darwin, tackled the problem from a different point of view. They 

 studied the distribution of deflection of alpha particles, shot out of 

 radioactive materials, as these alpha particles traveled thin sheets 

 of sohd materials. They found that tliis distribution was quanti- 

 tatively what would be expected if the deflections were produced by 

 ordinary electrostatic forces, varying as the square of the distance, 

 between the alpha paiticle and a very small object containing most 

 of the mass in each atom. They were thus able to show that this 

 small object is not more than one ten-thousandth of the diameter of 

 the atom, that it contained substantially all the mass of the atom and 

 that it carried a positive electric charge equal, in electronic units, to 

 about half the chemical atomic weight of the atom. 



Thus arose the concept that the atom is composed of a positive 

 nucleus of small dimensions, surroimded by electrons to the number 

 of about half the atomic weight. 



This had scarcely become established when it was brilliantly refined 

 and extended by Moseley, just before he went to his untimely death 

 in the Great War in 1914. Moseley had made a most ingenious study 

 of the spectra of X-rays of a large number of the chemical elements, 

 using a modification of the X-ray spectroscopy technique developed 

 by the Braggs. He found that the square roots of the frequencies of 

 the characteristic X-ray lines were numerically vcr}' simply related 

 to the number which gave the place of the element in the periodic 

 table of the elements, so useful to chemists but so entirely without 

 explanation. Thus this number acquired a definite physical signifi- 

 cance and is now well knowTi as the "atomic number." 



For all the elements heavier than hydrogen, this atomic number is 

 about half the atomic weight and, to make a long story short, tliis 

 atomic number turns out to be exactly the number of electronic units 

 of charge on an atomic nucleus, or the number of electrons in the 

 atom outside the nucleus. At the same time, Moseley's work proved 

 to be one of the greatest advances ever made in the basic interpretive 

 side of chemistry. 



Now that the number of electrons in each atom was kno^vn, the next 

 step was to wonder how they were arranged, what held them in place 

 and what they were doing in their spare time. Suggestions were not 

 slow in coming. In fact, even before Moseley's work, two rival theories 



