46 PRINCIPLES OF CHEMISTRY. 



Whilst the valence of some elements is invariably the same 

 under all circumstances, other elements show a different valence 

 (this means a different combining power for other atoms) under 

 different conditions. For instance : Phosphorus combines both 

 with 3 and 5 atoms of chlorine, forming the compounds PC1 3 

 and PC1 5 . As chlorine is a univalent element, we have to 

 assume that phosphorus has in one case 3, in another case 5 

 points of attraction. Many similar instances are known, and 

 will be spoken of later. 



The doctrine of the valence of atoms has modified our views 

 of the equivalence of atoms. We say the atoms of univalent, 

 bivalent, or trivalent elements are equivalent among each other ; 

 but two atoms of a univalent element are equivalent to one atom 

 of a bivalent element, or two atoms of a bivalent element to one 

 atom of a quadrivalent element, etc. 



After having explained this valence of atoms, it may now be 

 better understood why the atoms do not exist as such in a free 

 and uncombined state in an element, but combine with each 

 other to form molecules. The atoms having the tendency of 

 combining with, or attaching themselves to atoms, are bound to 

 exert that attraction, and if they are not combined with atoms 

 of other elements, they combine with each other. For instance : 

 Oxygen gas is not a mass of oxygen atoms, but of oxygen mole- 

 cules, each molecule being formed by the union of two atoms. 



QUESTIONS. 



51. State the law of the constancy of composition. 



52. What is the difference between a mixture and a compound ? 



53. Mention some instances of the production of molecular mixtures. 



54. State the law of multiple proportions. 



55. What considerations led Dalton to the adoption of the atomic 

 theory ? 



56. What regularity regarding volume is noticed when gases combine 

 chemically? 



57. To what was the term equivalent quantities applied formerly, 

 and what is to be understood by it to-day ? 



58. Explain the term quantivalence or atomicity. 



59. Mention some univalent, bivalent, trivalent, and quadrivalent 

 elements. 



