194 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



is recorded in the case of lithium, where it amounts to one-sixth. It 

 is about one-tenth in the case of boron, neon, argon, and krypton. 



The number of electrons in the nucleus tends to be even. — This rule 

 expresses the fact that in the majority of cases even atomic number 

 is associated with even atomic weight and odd with odd. If we con- 

 sider the three groups of elements, the halogens, the inert gases, and 

 the alkali metals, this tendency is very strongly marked. Of the 

 halogens— odd atomic numbers— all 6 (+1?) atomic weights are odd. 

 Of the inert gases— even atomic numbers 13 (+2?) are even and 3 

 odd. Of the alkali metals — odd atomic numbers — 7 are odd and 1 

 even. In the few known cases of elements of other groups the pre- 

 ponderance, though not so large, is still very marked and nitrogen 

 is the only element yet discovered to consist entirely of atoms whose 

 nuclei contain an odd number of electrons. 



If we take the natural numbers 1 to 40, we find that those not repre- 

 sented by known atomic weights are 2, 3, 5, 8, 13 (IT), (18), 21 (33), 

 34 (38). It is rather remarkable that these gaps, with the exception 

 of the four in parentheses, are represented by a simple mathematical 

 series of which any term is the sum of the two previous terms. 



In consequence of the whole number rule there is now no logical 

 difficulty in regarding protons and electrons as the bricks out of 

 which atoms have been constructed. An atom of atomic weight m 

 is turned into one of atomic weight m+i 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. 



The case of the element hydrogen is unique; 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 packed exceedingly closely together. Theory indicates that 

 when such close packing takes place the effective mass will be re- 

 duced, so that when four protons are packed together with two elec- 

 trons to form the helium nucleus this will have a weight rather less 

 than four times that of the hydrogen nucleus, which is actually the 

 case. It has long been known that the chemical atomic weight of 

 hydrogen was greater than one-quarter of that of helium, but so long 

 as fractional weights were general there was no particular need to 

 explain this fact, nor could any definite conclusions be drawn from 

 it. The results obtained by means of the mass-spectrograph remove 

 all doubt on this point, and no matter whether the explanation is to 



