1 6 AGRICULTURAL CHEMISTRY 



ical combination a definite number of atoms of other ele- 

 ments. Carbon, for example, has the power of uniting 

 with or holding in chemical combination four hydrogen 

 atoms ; carbon is said, therefore, to have a valence of 4. 

 Elements which have power to hold only one atom of hy- 

 drogen in combination are called monovalent. Hydro- 

 gen is a monovalent element. Bivalent, trivalent, tetra- 

 valent, and pentavalent elements are those whose atoms 

 have the power of uniting with 2, 3, 4, and 5 atoms 

 of hydrogen or other monovalent elements. The valence 

 of an element is spoken of as its combining power. Some 

 of the elements have more than one valence. The va- 

 lences of some of the more common elements are given 

 on page 1 1 . 



21. Combination of Elements. The combination of 

 two elements to form compounds is always governed by 

 the valence of the elements. When calcium and chlorin 

 combine, the combination takes place in a definite way ; 

 calcium has a valence of 2 ; chlorin has a valence of i ; 

 hence, in order to make a chemical combination, it will 

 take one atom of Ca, having a valence of 2, to combine 

 with two atoms of Cl, each Cl atom having a valence of 

 i. CaCl 2 is the formula. Calcium could not combine 

 with three atoms of chlorin, because compounds com- 

 posed of two elements are always formed according to the 

 valence of the elements. The valence of calcium, 2, lim- 

 its the number of atoms of chlorin with which it can com- 

 bine. If one of the elements, as oxygen, has a valence 

 of 2, and the other element, as carbon, has a valence of 

 4, 2 atoms of oxygen, each atom having a valence of 2, 



