36 SECTIONAL ADDRESSES. 



molecule and the products of the rupture of the link. Hence, the more 

 unstable these products are, the more difficult it is to break the link. The 

 rupture of a normal covalency leads to the production of two univalent 

 radicals 



A : B > A- + .B 



that is, of two highly unstable products. But a co-ordinate link can break 

 by the return of the two shared electrons to the atom to which they 

 originally belonged 



A:B y A + : B 



and one at least of the products is now a molecule capable of independent 

 existence. Thus, the products of the rupture of a co-ordinate link are, as 

 a rule, more stable than those formed by breaking a normal covalency, 

 and the co-ordinate link is therefore less stable. This difference is 

 particularly marked in rings containing co-ordinate links, those which 

 Prof. Morgan has called chelate rings : these are far more sensitive to 

 strain, owing to the weakness of the co-ordinate link, than the ordinary 

 rings of organic chemistry ; while the latter are known of every size from 

 three to eighteen members, chelate rings almost invariably contain 

 either six or five ; a few 4-rings are known and one or two 7- and 8-rings ; 

 but none with less than four or more than eight members. This 

 explanation of the difference in strength between normal and co-ordinate 

 links is of considerable importance ; the fact is beyond dispute, and if 

 we are to maintain that the mechanism of both forms of linkage is the 

 same, consisting in the sharing of two electrons, we must be able to give 

 a reason for this difference in stability. 



The second point of difference is that while the normal covalency in- 

 volves no considerable disturbance of the electrostatic equilibrium in the 

 molecule, this is not true of the co-ordinate link. In the normal link 

 between two atoms, each atom shares one electron with the other atom. 

 If the electrons were shared equally between the two, there would be no 

 electrostatic disturbance at all. We do not know enough about the 

 dynamics of the sharing of electrons to say how nearly this is true, but 

 the properties of ordinary covalent compounds indicate that it is not far 

 from the truth, and that the shared electron usually divides its time 

 more or less equally between the two atoms which share it. But when a 

 co-ordinate link is formed between two originally neutral atoms, one of 

 them loses and the other gains a share in two electrons. Hence, the 

 acceptor must receive a negative charge from the link and the donor a 



positive charge. This fact is expressed by some chemists, such as Prof. 



+ — 

 Lowry, by writing the link A — B instead of A->B. A molecule contain- 

 ing such a link is therefore an electrical dipole. This electrostatic 

 disturbance will have two chief results : it will increase the dielectric 

 constant of the substance, and it will increase the attraction of the 

 molecules for one another, and therefore diminish the volatility. That 

 this does actually occur we have plenty of evidence ; I may give a few 

 examples, selected from non-associated substances, in order to avoid 

 the complications which association might produce. While the value 

 of the dielectric constant for hydrocarbons is about 2-3, for ethers about 4. 



