POLARITY OF MATTER 49 



This exhibition of polarity by the elements is 

 called their valency or chemical affinity ; and the 

 elements are unipolar, bipolar, or multipolar, accord- 

 ing as each atom has the power of attracting or 

 combining with one or more atoms. It is as though 

 each atom were a magnet, with one or more poles 

 by which other atoms are attracted or attached to 

 it. These valencies or poles have a strong craving 

 for satisfaction, called saturation ; so great is their 

 attraction that they seize upon their own kind when 

 no other element is free, or they will displace atoms 

 from some other combination to form molecules. 



The valency of the ions may therefore be repre- 

 sented as the poles of attraction, and each ion is a 

 monad, dyad, triad, tetrad, or pentad, according to 

 the number of poles of attraction, valencies, or 

 affinities it exhibits. Arranged in this manner the 

 ions can be classified according to the following 

 examples : 



A. Cations. 



1. MONADS : Monovalent or unipolar ions e.g., 



H (in acids), NH 4 , K, Na, Li, 

 Ag, also Hg (mercurous) and 

 Cu (cuprous). 



2. DYADS : Divalent or bipolar ions, as Mg, Ca, 



Fe (ferrous), Ba, Sr, S, Zn, Pb, 

 also Cu (cupric) and Hg (mer- 

 curic). 



3. TRIADS : Trivalent or tripolar ions, as Fe 



(ferric), Al, Bi, Sb. 



