450 Dr. S. H. 0. Briggs: The Elements 



two charges and is combined with two electrons to give the 

 element X A E 2 , each electron will be united to the kernel by 

 w lines of force where w>v, because the attractive force due 

 to the second charge is distributed equally over the spherical 

 surface, and therefore that portion of the sphere directly 

 under the influence of the first electron can bind more lines 

 of force than in the atom X A E, in which the nucleus had 

 only one charge. 



Consequently, as the charge on the nucleus is increased 

 and more and more valency electrons are added, the degree 

 of saturation of each electron increases, until a stage is 

 reached when the whole of the spherical surface is brought 

 under the influence of the electrons, and the nucleus and 

 electrons are all completely saturated. This will take place 

 when the electrons are sufficiently numerous, and are distri- 

 buted symmetrically in space around the sphere. According 

 to the theories of Lewis and Langmuir, a condition of this 

 kind cannot occur until eight electrons are united to the 

 kernel, as in neon. 



If the formula X A E represents the lithium atom, we there- 

 fore find as we pass along the series Li, Be, B, C, N, 0, F, 

 Xe, that with increase in the nuclear charge, the valency 

 electrons are more and more firmly held, and the electro- 

 positive character (tendency to lose electrons) of the atoms 

 diminishes, until in neon in which each electron is completely 

 saturated it vanishes altogether. 



The gradation in properties from lithium to neon is 

 exactly analogous to the change observed as we pass along 

 the series LiCl^ BeCl 2 , BC! 3 , CC1 4 , from the strongly polar 

 lithium chloride to the truly non-polar carbon tetrachloride 

 (see Trans. Chem. Soc. cxv. p. 278 (1919)). The parallelism 

 is clearly brought out by Langmuir's description of lithium 



the strongest possible electrolyte where v=0 to the truly non-polar 

 compound where v = u. In the non-polar compound, however, no 

 transfer of electrons has occurred, but the electrons are shared equally 

 between A and B. In writing the formula of the non-polar compound 

 w T e therefore use two equal arrows pointing in opposite direction, i. e. 

 A^I±B. 



Positive affinity is defined as a tendency to lose electrons and negative 

 affinity as a tendency to attract electrons. The potassium atom and the 

 chlorine atom have therefore positive and negative affinity respectively, 

 w^hereas the potassium ion and the chloride ion have negative affinity 

 and positive affinity. The affinity of the atoms is termed primary affinity 

 and that of the ions secondary affinity. Potassium is therefore an 

 element with primary positive affinity and secondary negative affinity, 

 and chlorine is an element with primary negative affinity and secondary 

 positive affinity. 



