1 1 6 CHEMISTR V FOR A GRIC UL TURAL STUDENTS 



Aluminium appears to be trivalent, for it forms the compound 

 -^^2 '"^s"- Frequently the elements have more than one valency, 

 for iron forms ferrous and ferric chlorides, Fe"Cl2' and Fe'" CI3'. 

 Groups of elements act like elements. Thus NH^, OH and 

 NO3 are univalent like K and CI, forming (NH4)'C1', Na'(OH)' 

 and H^NOg)' ; CO3 and SO^ are bivalent, forming Na2'(C03)" 

 and Ca"(S04)"; PO4 is trivalent, forming H3'(P04)"' and 

 Ca3"2(P04)"'; SiO^ is quadrivalent, forming Na/i^SioJ"". 



This study of a few of the more important elements has shown 

 that they fall into groups having certain properties in common. 

 Of these groups may be mentioned potassium and sodium ; 

 calcium, strontium, and barium ; zinc and magnesium j chlorine, 

 bromine, and iodine ; oxygen and sulphur ; nitrogen and phos- 

 phorus. The properties common to the elements of each group 

 are both physical and chemical, e.g. softness, density, oxidisa- 

 bility, solubility of salts, and valency, and therefore the composi- 

 tion of compounds. When three or more elements exist in a 

 group, it is found that the properties vary as the atomic weights. 

 Thus strontium has an atomic weight intermediate between 

 those of calcium and barium ; its properties, and the properties of 

 its compounds, are also intermediate. Bromine has an atomic 

 weight intermediate between those of chlorine and iodine; 

 chlorine is a gas, iodine a solid, and bromine a liquid. When 

 all the known elements are arranged in the order of their 

 atomic weights, it is found there is a periodic recurrence of 

 similar properties, so that if each period be placed one under 

 the other, the elements fall into groups (the elements men- 

 tioned above are arranged as they arrange themselves in these 

 groups); and it is possible even to foretell the properties of 

 any element by its atomic weight, and the position it would 

 therefore assume in relation to other elements. These facts 

 are mentioned, because it should be realised that the reactions 

 and properties of the substances studied in the foregoing 

 pages are not haphazard, but the result of this periodicity of 

 properties depending on the atomic weights. 



