LAWS OF CHEMICAL COMBINATION. 45 



We see here, that Cl, Br, and I combine with H in the pro- 

 portion of atom for atom ; O, S, Se combine with H in the pro- 

 portion of 2 atoms of hydrogen for 1 atom of the other ele- 

 ment; N", As, P combine with 3; C and S with 4 atoms of 

 hydrogen. 



It has been, moreover, found that the compounds mentioned 

 in column I. are the only ones which can possibly be formed by 

 the union of the elements Cl, Br, and I with H. They inva- 

 riably combine in this proportion only. Other elements show a 

 similar behavior. For instance, the metal sodium combines 

 with chlorine or bromine in one proportion only, forming the 

 compounds NaCl or NaBr. 



Looking at columns II., III., and IV., we see that the elements 

 mentioned there combine with 2, 3, and 4 atoms of hydrogen, 

 respectively. It is evident, therefore, that there must be some 

 peculiarity in the power of attraction of different elements to- 

 wards other elements, and to this property of the atoms of ele- 

 ments of holding in combination one, two, three, four, or more 

 atoms of other elements the name atomicity, quantivaleiice, or 

 simply valence, has been given. 



According to this theory of the valence of atoms, we dis- 

 tinguish univalent, bivalent, trivalent, quadrivalent, etc., ele- 

 ments. All elements which combine with hydrogen in the 

 proportion of one atom to one atom are univalent, as, for in- 

 stance, Cl, Br, I, Fl, and all elements which combine with these 

 in but one proportion, that is, atom with atom, bear the same 

 valence or are also univalent, as, for instance, Na, K, Ag, etc. 



Those elements which combine with hydrogen or other univa- 

 lent elements in the proportion of one atom to two atoms are 

 bivalent, such as 0, S, Se. 



Trivalent and quadrivalent elements are those the atoms of 

 which combine with 3 or 4 atoms of hydrogen, respectively. 

 Figuratively speaking, we may say that the atoms of univalent 

 elements have but one, those of bivalent elements two, of triva- 

 lent elements three, of quadrivalent elements four points of 

 attraction, by means of which they may attach themselves to 

 other atoms. 



To indicate the valence of the elements, dots or numbers are 

 frequently placed above the chemical symbols, thus H', O n , N !li , 

 C iiH or C iv . 



