Influence of Atomic Weight. 



369 



the normal halogen compounds of the elements are a 'periodic 

 function of the atomic weights of the constituent elements. For 

 if the elements be arranged in the order of their atomic 

 weights, then the melting-points, boiling-points, and heats of 

 formation of their halogen compounds rise and fall periodically 

 nine times, these periods corresponding exactly with Mendel- 

 jeff's nine series of elements. The maxima occur at the posi- 

 tive and the minima at the negative end of each series. 



(2) The influence of the halogen on these same physical pro- 

 perties increases with the number of its atoms in the compound, 

 thus : — 



Boiling-point 

 I Br = 390* 

 I CI =373 



17 



Boiling-point. 

 SbBr 3 = 549 

 SbCl 3 =496 



53 



Boiling-point. 

 SnBr 4 = 474 

 Sn Cl 4 =388 



86 



(3) In any normal halogen compound the influence of either 

 of the elements on the melting- or boiling-point increases ivith its 

 own atomic iveight, and decreases with the atomic iveight of the 

 other element. Thus : — 



Melting-point. 

 SbBr 3 =363 

 AsBr 3 =295 



68 



Melting-point. 



Sbl 3 =438 

 SbBr 3 = 363 



75 



In each of the above pairs of compounds which contain one 

 element in common the melting-point increases with the 

 atomic weight of the variable element, and that whether the 

 latter be the positive or negative constituent — thus showing 

 that the influence of an element on the melting-point of one 

 of its compounds increases with its own atomic weight. That 

 it decreases with the atomic weight of the other element is 

 shown by the following examples : — 



Melting-point. 

 Asl 3 =419 

 AsBr 3 = 295 



124 



Melting-point. 

 Sbl 3 =438 

 SbBr 3 =363 



75 



Here the substitution of I for Br produces a larger increase 

 in the melting-point in the case of the As than in that of the 



* These numbers are all reckoned from the absolute zero - 273° C, and 

 this is the case with all melting-points and boiling-points subsequently 

 referred to. 



