THE PHYSICIST'S PRESENT CONCEPTION OF AN ATOM 



By R. A. M1U.IKAN 

 Norman Bridge Laboratory of Physics 



All scientists agree upon an atom which has a very minute posi- 

 tively charged nucleus surrounded in its outer regions by a number 

 of negative electrons just sufficient to neutralize the free positive 

 charge upon the nucleus. 



We all agree that the number of these positive charges upon the 

 nucleus varies from 1, in the case of hydrogen, by unit steps up 

 to 92 in the case of uranium, and hence that the number of negatives 

 held in the outer regions also varies from 1 to 92. 



We all agree that the chemical properties of all atoms, and most of 

 the physical properties, too, mass being the chief exception, 'are de- 

 termined simply by the number of these electrons; primarily by the 

 number of them which are found in the outermost shell and which 

 we call the valence electrons. 



We all agree, too, that the nucleus is extraordinarily minute, so 

 that if all the dimensions of an atom were magnified ten billion 

 times — ^^a magnification which would make a bird shot swell to the 

 size of the earth and would make the diameter of the atom about a 

 meter — the nucleus, on this huge scale of magnification, would not 

 be more than a tenth of a millimeter in diameter — that is, not larger 

 than a mere pin point. 



We all agree, too, that in the case of uranium there are packed into 

 that infinitesimal nucleus 238 positive and 146 negative electrons, the 

 exact number of positives being determined simply by the atomic 

 weight, while the number of negatives which bind the positives is the 

 atomic weight minus the atomic number. This obviously me'ans that 

 both positive and negative electrons are so infinitesimally small that 

 for practical purposes we may ignore their dimensions altogether and 

 think of them as mere point charges. 



We all agree that so far as physical science has now gone there 

 have appeared but these two fundamental entities, namely, positive 



' An address delivered before the sixty-seventh convention of the American Chemical 

 Society, Washington, D. C, Apr. 22, 1924. Reprinted by permission from Science. Vol. 

 LIX, No. 1535, May 30, 1924. 



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