46 PRINCIPLES OF CHEMISTRY. 



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

 of atom for atom ; O, S, Se combine with H in the proportion of 2 

 atoms of hydrogen for 1 atom of the other element ; N, As, P com- 

 bine with 3 ; C and Si with 4 atoms of hydrogen. 



Moreover, it has been found that the compounds mentioned in 

 column I. are the only ones which can be formed by the union of 

 the elements Cl, Br, and I with H. They invariably 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 compound 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 pecu- 

 liarity in the power of attraction of different elements toward other 

 elements, and to this property of the atoms of elements of hoi rh' 110- 

 in combination one, two, three, four, or more atoms of other ele- 

 ments the name atomicity, quantivalence, or simply valence, has been 

 given. 



According to this theory of the valence of atoms, we distinguish 

 univalent, bivalent, trivalent, quadrivalent, quinquivalent, sexivalent, 

 arid septivalent elements. ^11 p f lpmpnts which combine_with hydro- 

 gpnrn^fVip proportion of one atom to one atomare univalent, as, for 

 instance, Cl, Br, I, F, 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 univalent 

 elements in the proportion of one atom to two atoms are bivalent, 

 such as O, 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 trivalent elements three, of 

 quadrivalent elements four bonds or points of attraction, by means of 

 which they may attach themselves to other atoms. 



C Elementary atoms are often named according to their valence !| 

 nonads, diads, triads, tetrads, pentads, hexads, and heptads. / 



To indicate the valence of the elements frequently dots or numbers 

 are placed above the chemical symbols, thus IT, O u , N li! , C mi or C iv . 

 The bonds are often graphically represented by lines, thus : 



H- -0-, 



N , _C 



