274 



The question naturally arises : can we group all of the elements 

 into families like the halogen family and the sulphur family? 

 If so, then we shall lighten considerably the burden of chemical 

 facts that we need to remember, for the behavior of one element 

 in a family will suggest to us immediately how the other members 

 of the same group will act under similar conditions. Classifica- 

 tion of this kind is part of the method of science, and furnishes 

 a very useful guide in investigation. 



Metallic and Non- Metallic Elements. Thus far we have 

 found the division into metallic and non-metallic elements very 

 serviceable for classification in terms of chemical relations. The 

 metallic or positive elements (p. 54), form positive radicals 

 and ions containing no other element (cf. p. 171). Thus 

 the metals give sulphates, nitrates, carbonates, and other salts, 

 which furnish a metallic ion, such as Na + or K + , together with 

 the ions SO 4 =, NOr, and C0 3 =. Their hydroxides, KOH, Ca- 

 (OH 2 ), etc., give the same metallic ion, and the rest of the mole- 

 cule forms hydroxide-ion. That is to say, their hydroxides are 

 bases and their oxides are basic. The metallic elements often 

 enter, but only with other elements, into the composition of a 

 negative ion, as is the case with manganese in K.Mn0 4 , with chro- 

 mium in K 2 .Cr 2 07, and with silver in K.Ag(CN)2. 



The non-metallic or negative elements are found chiefly in 

 negative radicals and ions. They form no nitrates, sulphates, car- 

 bonates, etc., for they could not do so without themselves alone 

 constituting the positive ion. We have no such salts of sulphur, 

 carbon, or phosphorus, for example. Their hydroxides, although 

 their formulae may be written C1O 2 OH, P(OH) 3 , S0 2 (OH) 2 , fur- 

 nish no hydroxyl ions, as this would involve the same conse- 

 quence. These hydroxides are divided by dissociation, in fact, so 

 that the non-metal forms part of a compound negative radical, and 

 the other ion is hydrogen-ion, C10 3 .H, P0 3 H.H 2 , S0 4 .H 2 . Their 

 oxides are acidic. Their halogen compounds, like PBr 3 (p. 201) 



