58 PRESIDENTIAL ADDRESS SECTION B. 



crowded together on the surface of the nuclei of the higher elements 

 and are thrown off in pairs, X 2 being the a-particle. It is thus the 

 accumulation of X rather than that of positive charge which causes 

 radioactivity. 



To repeat, the contention I wish to make is that the physicists, 

 as far as I understand them, have placed too much emphasis on the 

 fact that chemical properties depend on the valency electrons, and 

 have not considered the possibility that the arrangement of the 

 valency electrons themselves probably depends on the prominence 

 of certain points in the nucleus, e.g., that a tetrahedral arrange- 

 ment of valencies in CH d and NH 4 + connotes a tetrahedral nucleus 

 for C and N. 



The second point concerns Moseley's law for the K-lines in 

 X-ray spectra. The expression X /A = K(N— l) 2 becomes 1 /\ = K.2 2 

 for lithium. If we make the simple assumption that H+ is a 

 nucleus and the only free nucleus known (omitting the products of 

 radioactivity) hydrogen becomes an entity sui generis, and the first 

 real ordinary atom is helium. Moseley's law then becomes 

 l l X =K X (N+l) 2 , in which N" is zero for helium and is not the 

 atomic number but the number of valency electrons in the smaller 

 elements. For higher elements N is the number of electrons out- 

 side the kernel. 



For the L series of lines in the higher elements it is possible 

 that the law is that 1 /X varies as the square of the atomic weight 

 (not number) counting neon as zero number (i.e., subtracting 20), 



viz., 1 /A=C(A-20) 2 instead of l /X= -A. K (N-7.4) 2 as given 



A i 



by Moseley. 



Some account of Gilbert Lewis's octet theory (J. Amer. Chem. 



Soc, 1916, 768) which is almost certainly true for the elements 



higher than oxygen, may now be given. Fluoride ion, neon and 



sodium ion all have the same octet of electrons; but in fluorine atom 



one is missing, leaving a "hole." In sodium metal one is in excess. 



In sodium fluoride the extra electron of sodium metal fills the hole 



in the fluorine atom : both then become ions even in the crystal 



state, and both have complete octets, thus: — 



[46 [Xa+ 9 ] 46] 6 -f 46 [F+ 7 ] 30 = [46 [Na+ 9 ] 46] [46 [F+ 7 ] 46] 



Metal : kernel Sodium ion Fluoride ion 



and electron Sodium Fluoride crystal. 



Note. — The expression Na+ 9 is really Na +11 - 2 , the atom having 

 2 inner electrons to make a kernel out of a nucleus (see page 51). 

 The fundamental conception might appear (reading Lewis strictly) 

 to be that the same electron belongs to two atoms when a compound 

 is formed, but undoubtedly in the ionisable compounds the metal 

 electron is transferred and the residue, which is the me.tal ion, is 

 quite independent not only in solutions but in crystals, the atoms 

 in which are simply held together because one ion is positive as a 

 whole and the other negative as a whole. In the case of such a 

 substance as methyl chloride, the hydrogens are held in the non- 

 ionisable manner (see p. 50), but the chlorine is partly present as 

 chloride ion, since powerful reagents like silver nitrate can remove 



