NON-MATHEMATICAL SCIENCES 173 



has two, the nitrogen atom (N) usually has 

 three, and the carbon atom (C) has four. Thus 

 hydrogen can be attached to only one other 

 atom, oxygen to two 



others, and so on. By h- -o m- -c- 



means of various chemi- 

 cal reactions it is pos- h h 

 sible to make one atom h-o~m h-h-h h-c-h 

 or group take the place " 



» ,1 T . , Figure 26 



oi another, and m most . . . xt , 



' ^ ^ Atomic Valences 



such substitutions it has 



proved justifiable to assume that the structure 



of the rest of the molecule is unaffected. In this 



way it is possible to build up or to synthesize 



complex molecules of known composition. 



Sometimes two mole- 



y y V V cules, as shown in Fig- 



H-c-c-o-H H-c-o-c-H ^^^ gg^ ^^^^ ^^ie samc 



atoms but in a differ- 



FlGUEE 26 , 



T ent arrangement, and 



Isomers & ' 



the two are known as 

 isomers. The number of possible isomers that 

 can be prepared from a given set of atoms in- 

 creases with enormous rapidity as the number 

 of the atoms in the set increases. This number 

 of isomers not only soon exceeds the total num- 

 ber of known substances, but a molecule with 



