104 PRINCIPLES OF CHEMISTRY. 



tions. 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. 



An explanation which is sometimes given in regard to the variability of 

 the valence of atoms is the assumption that sometimes one or more of the 

 bonds of an atom unite with other bonds of the same atom. If, for instance, 

 in the quinquivalent phosphorus atom two bonds unite with one another a 

 trivalent atom will remain. 



It is noticed that the valence of atoms in nearly all cases increases or di- 

 minishes by two, which could not be otherwise, if the explanation given be 

 correct. Thus chlorine, the valence of which generally is I., may also have a 

 valence equal to III., V., or VII., while sulphur shows a valence either of II., 

 IV., or VI. Atoms whose valence is even, as in the case of sulphur, are called 

 artiadu ; those whose valence is expressed in uneven numbers, as chlorine and 

 phosphorus, are called perissads. 



While it is now being assumed that most of the elements possess more than 

 one valence, in consequence of the assumed power of bonds in the same atom 

 to saturate one another, in this book will be mentioned chiefly that valence 

 which the element seems to possess predominantly. 



The doctrine of the valence of atoms has modified our views of the 

 equivalence of atoms. We now say that all atoms of a like valence 

 are equivalent to each other. The atoms of each univalent element 

 are equivalent to each other, and so of the atoms of any other valence, 

 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. 



QUESTIONS. Define a chemical change and state the various modes of 

 effecting it, with examples. Define element, compound, combination, and 

 decomposition. About how many elements exist? What is chemical energy? 

 Define exothermic and endothermic actions. What is chemical affinity and 

 how does it differ from other forces? State the law of constancy of composi- 

 tion. What is the distinction between a mixture and a chemical compound? 

 Give examples of each. State the law of multiple proportions. Give in full 

 Dalton's atomic theory and show how it accounts for the laws of combination. 

 What is the relation of atoms to molecules? Define atomic and molecular 

 weight. What atom is chosen as the unit of atomic weights, and why? What 

 are chemical symbols and what do they signify? Calculate the per cent, of 

 oxygen and hydrogen in water, H 2 O. What weight of carbon dioxide, CO,, 

 would result from 25 grammes of carbon? What regularity regarding volume 

 is noticed when gases combine? Define valence. What were considered for- 

 merly as equivalent quantities, and what are such at present? Mention some 

 univalent, bivalent, trivalent, and quadrivalent elements. What explanation 

 is offered for variable valence ? 



