30 SECTIONAL ADDRESSES. 



the number of places by which an element was removed from an inert 

 gas, and was positive if it came after the inert gas, and negative if it 

 came before. The more difficult problem of the non-ionised link, such 

 as we find in elementary chlorine or hydrogen, or in methane, was 

 explained by Lewis by the assumption that it is possible for two atoms, 

 each of which is a few electrons short of a stable number, to share electrons 

 in such a way that each counts as part of the constitution of each atom, 

 thus forming a link which is not merely due to electrostatic attraction, 

 and so cannot be ionised. 



These views of the two fundamental kinds of linkage — ionised and 

 non-ionised, polar and non-polar, or, as Langmuir has conveniently 

 called them, electrovalent and covalent — that one is due to the trans- 

 ference and the other to the sharing of electrons between two atoms, have 

 been confirmed by all subsequent discoveries, and may be taken to be 

 generally accepted. The atomic models on which both Kossel and Lewis 

 founded their theories have indeed been shown to be impossible. These 

 authors supposed that the electrons surrounding the nucleus were at 

 rest, and Lewis in particular assigned to them definite positions in his 

 famous cube, which was subsequently developed in so much detail by 

 Langmuir. We now know that any such static hypothesis is untenable ; 

 it involves the assumption of a variety of otherwise unknown forces, and 

 it is incapable of explaining many of the properties of atoms, especially 

 their spectra ; whereas all these are accounted for by a dynamic model, 

 in which the electrons move in orbits round the nucleus much as the 

 planets move round the sun. But the conceptions of the transference 

 and the sharing of electrons can equally well be applied to the dynamic 

 model of Bohr. 



So far the mechanism of valency at which we have arrived is that of 

 structural chemistry rather than that of co-ordination. The numerical 

 value of the valency of an atom appears equal to the excess or defect of 

 its electrons as compared with the stable number of an inert gas. If it 

 has, say, two electrons in excess, loosely attached and forming an imperfect 

 group, it can lose them and become a divalent cation, or it can share them 

 and so form two covalent links ; if it has two electrons less than the 

 stable number it can take up two from another atom or atoms and become 

 a divalent anion, or it can share two electrons belonging to other atoms 

 and become di-covalent ; if the excess or defect is two, the valency, of 

 whichever kind, is two also. The next element, with an excess or defect 

 of one, will have a valency of one. We thus arrive at the relation between 

 the valency of an element and its group in the periodic table which was 

 originally pointed out by MendeleefE. In fact the majority of the structural 

 formulae of organic chemistry can be translated into electronic formulae 

 by the simple process of writing two dots (for two shared electrons) in 

 place of a line. It is important to notice the reason for the two dots — 

 for Lewis's assumption that two shared electrons are necessary for every 

 covalency. The most familiar property of valency, which has been recog- 

 nised from the earliest times, is that if one atom combines with another 

 it not only uses up one of its own units of combining power, but one of 

 those of the other atom as well. Where the link is ionised, the reason 

 of this is obvious : the electron which one atom loses must be taken up 



