PRESIDENTIAL ADDRESS SECTION B. 51 



nuclear charge of 4 and a total of 4 external electrons. (See Fig. 4.) 

 Sodium is 8 places higher than lithium, therefore has a nuclear 

 charge of 11 (3 + 8) and 11 external electrons. The nuclear charge of 

 + 11 is the difference between 23 hydrions (giving the weight) and 12 

 electrons; and as regards the eleven external electrons it is assumed 

 that 10 of them are close to the nucleus, forming along with the 

 nucleus the "Kernel" (or "ion") of sodium. Thus Na ' (the ion) 

 is 50(12H+ 120 llH+)50, and sodium metal is this kernel with a 

 separate electron at a greater distance, as in the case of lithium. 

 In order to bring out the complete analogy between sodium and 

 lithium it is assumed that the 10 external electrons are arranged 

 so that 2 of them are just outside the nucleus and the other 8 are 

 about twice as far away, all equidistant from the centre and there- 

 fore arranged something like the 8 corners of a cube round the 

 nucleus. This arrangement of 8 electrons, the conception of which 

 is due to Gilbert Lewis, and which is technically called a "com- 

 pleted octet," is assumed from its perfect symmetry to confer 

 chemical inertness, and as a matter of fact sodium ion, except 

 towards electricity, is quite as inert as argon, going through all 

 sorts of chemical actions unchanged. Similarly potassium ion K " 

 is 90(2OH+2O0 19H+)90, in which the nucleus with 19 plus charges 

 (potassium is 8 above sodium, i.e., is the 19th element) has 18 

 external electrons, viz., the arrangement of 10 which exists in 

 sodium, along with another "completed octet," thus giving again 

 a kernel or ion which has no projections and is therefore inert. 



When we come to the next member rubidium we find that the 

 atomic weight is no longer twice the atomic number plus one as it 

 is in the case of lithium, sodium and potassium. Rubidium ion 

 Rb' is 180(4811+480 37H+)180 in which the nucleus contains 10 

 more hydrions and electrons than would have been expected from 

 the lower members: the nuclear charge is 37, and there are 36 

 external electrons, arranged counting from the inside as before as 

 2, 8, 8, 18. Irving Langmuir (J. Amer. C.S. 1919, 879) suggests 

 that the external 18 are arranged not like the internal 18 {viz., 

 as 2, 8, 8) but by uniform distribution over a spherical surface, 

 viz., two polar and 8 in each of two zones round the equator. This 

 again gives a surface without projections or lacunae, and corres- 

 ponds to the inertness of the rubidium ion. Similarly Cs " 

 (caesium ion) is 270(7811+780 55H+)270, with weight 133, nuclear 

 charge 55, and 54 external electrons arranged 2, 8, 8, 18, 

 18. The top member (which is unknown) "ekacaesium" (it 

 lies between niton and radium) would have the structure 

 430(136H+1360 87H+)430, with external electrons arranged 2, 8, 

 8, 18, 18, 32. This arrangement of the electrons in concentric 

 layers finds its origin in Rydberg's remarkable observation that the 

 atomic numbers of helium, neon, argon, krypton, xenon and niton 

 {viz., 2, 10, 18, 36, 54, and 86) which are also the numbers of 

 external electrons in the ions of lithium, sodium, potas- 

 sium, rubidium, caesium and ekacaesium, are given by the 

 mathematical formula N=2 (l 2 + 2 2 + 2 2 + 3 2 + 3 2 -fi 2 , etc.) e.g., 

 36 = 2(P + 2 2 + 2 2 + 3 2 ) for the fourth member. This at once gives 



