B.— CHEMISTRY. 33 



atom which lends the electrons (A) the donor, and that which receives 

 them (B) the acceptor. 



We have now to apply these ideas to the compounds on which Werner 

 based his theory. Any simple cation — that is, an atom stripped of its 

 valency electrons — can act as an acceptor. It can build up a valency 

 group by sharing electrons belonging to other atoms, that is, by forming 

 co-ordinate links. Thus the chromic ion [Cr] + + + contains a stable core 

 of twenty-one electrons and has no valency group ; the stability of this 

 arrangement is proved by the stability of the chromic salts. This ion 

 can then form a series of co-ordinate links with molecules of ammonia, 

 by sharing the ' lone pair ' of electrons of the nitrogen atom. Since the 

 stable size of the valency group for such an ion is 12, six molecules of 

 ammonia will be taken up, and in this way the hexammine [Cr(NH 3 ) 6 ]Cl 3 

 is produced. We have thus accounted for the power which certain 

 complete molecules possess of combining further through co-ordination. 



The next point is to explain the peculiar change of electrovalency 

 which accompanies the replacement of an ammonia molecule by, say, a 

 chlorine atom. It is natural that if an ammonia molecule is removed, 

 this should be replaced by another covalently linked atom, because that 

 is required to maintain the valency group of 12. When the ammonia is 

 removed it takes away with it the two shared electrons which it originally 

 contributed ; the chlorine atom which replaces it supplies one electron 

 to be shared by the chromium, but the chromium is called upon to supply 

 the other electron for the link. Thus the chromium is one electron short 

 of its stable number, and must take up an electron from elsewhere to make 

 up the deficiency. In other words, the replacement of the ammonia by 

 chlorine will reduce the positive charge on the ion bv one unit, giving 

 instead of [Cr(NH 3 ) 6 ] + + + the ion [Cr(NH 3 ) 5 Cl] + + , or the salt[Cr(NH 3 ) 5 Cl]Cl 2 . 

 The same change will occur for every replacement of a whole molecule in 

 the complex by a univalent radical. Thus the very peculiar change of 

 electrovalency which Werner established is a necessary result of the 

 electronic mechanism underlying the linkage. The third important 

 characteristic of the co-ordination compounds is the co-ordination number 

 itself. As we have seen, the most remarkable point about these com- 

 pounds is that the relation observed in ordinary structural chemistry 

 between the valency of an element and its group in the periodic table 

 disappears. Instead of finding that the valency — the number of links 

 which an atom can form — increases from one in the first group to four in 

 the fourth, and then falls (in the simpler compounds at any rate) to one 

 in the seventh, we find that the co-ordination number is independent of 

 the periodic group, and is usxially either six or four. But this again 

 follows necessarily from the theory. So long as the valency is expressed 

 by ionisation, or by normal covalencies to which each atom contributes 

 one electron, it must be limited either by the number of electrons which 

 the atom has to offer or by the number for which it has room in its valency 

 group ; it will therefore be determined by the distance of the atom in 

 question from the nearest inert gas, or, in other words, by the group in 

 the periodic table to which it belongs. In its saturated compounds the 

 atom will usually be left either with an imperfect valency group (like 

 the boron in boron trichloride) or with one which is incompletely shared, 



1927 D 



