32 SECTIONAL ADDRESSES. 



combination shown by apparently saturated molecules such, as water 

 and ammonia, the attainment of a valency limit (the co-ordination number) 

 independent of the periodic group to which the atom belongs, and the 

 peculiar change of electrovalency which accompanies the replacement of 

 a univalent radical such as chlorine by a whole molecule such as ammonia.. 

 We may consider these in turn. In nitrogen there are five valency elec- 

 trons ; by combination with three hydrogen atoms this number is increased 

 to eight, giving a molecule of ammonia, in which the octet of the nitrogen 

 is complete and the atom is so far saturated. But, though complete, the 

 octet is not fully utilised : six of its members are shared with the three 

 hydrogen atoms, but the other two are unshared, and so can form a fourth 

 link if another atom can be found which will share them without sharing 

 some of its own electrons with the nitrogen in return. This may happen 

 in a variety of ways. A hydrogen ion, consisting of a single proton with 

 no attendant electron, is capable of taking up two electrons, and, as we 

 all know, if a hydrogen ion meets an ammonia molecule it combines with 

 it to form an ammonium ion 



H 



H:N + [H] + = 



H 

 H:N:H 



H L H 



The nitrogen has now shared all its eight valency electrons, two with 

 each of the four hydrogen atoms ; but since the ammonia molecule is- 

 electrically neutral, while the hydrogen ion is positively charged, the 

 resulting NH 4 molecule is also positively charged. Again, boron has 

 three valency electrons ; it can share one of them with each of three 

 chlorine atoms (thus completing the octets of the chlorines), and at the 

 same time take a share in one of the electrons belonging to each of the 

 chlorines. This gives boron trichloride BC1 3 , in which the boron has 

 increased its valency group from three to six. The boron cannot combine 

 with a fourth chlorine atom, because, although its own octet is not complete, 

 it has no more unshared valency electrons to offer for a covalent link. 

 But if it meets an ammonia molecule it can share the unshared pair of 

 electrons of the nitrogen, and so form a co-ordinate link : — 



CI 

 CI : B + 



CI 



In this way each of the two atoms assumes a covalency (or, if we prefer 

 to call it so, a co-ordination number) of four. 



The conditions for the formation of a co-ordinate link thus are that 

 we should have one atom which has a pair of unshared valency electrons 

 to offer, and another which has room for one or more pairs of electrons 

 in its valency group. It is convenient to have a symbol and a nomen- 

 clature to express this process, and I have therefore suggested that, while 

 the ordinary covalent link is represented by a line A — B, the co-ordinate 

 link should be written as an arrow A-*B, pointing away from the atom 

 which contributes the two electrons of the link ; also we may call the 



