INTRODUCTION. 11 



trate the sphere of influence of another atom or molecule. On the other 

 hand the electrons are very small and compare in relative size to the atom 

 much as the sun compares in size to the solar system. Electrons can, there- 

 fore, move through ions and atoms or can move in the interatomic spaces 

 with considerable ease. In the metals a large number of electrons are free. 

 In organic compounds that are transparent to certain wave-lengths the 

 electrons, in general, will be held within certain regions by forces that are 

 elastic in their nature; that is, the force increases in proportion to the 

 amount the electron is moved from its position of equilibrium. 



THE THEORY OF CHROMOPHORES. 



In considering absorption spectra it is often quite sufficient to speak 

 qualitatively of the color of different compounds. The introduction of cer- 

 tain groups into colorless compounds often results in a colored compound. 

 Any such group is a chromophore. Sometimes the chromophore may be 

 weak and it may require the addition of several chromophores to produce 

 a colored compound. Ultimately the color is due of course to absorbers 

 existing within the chromophore. Among the better known chromophores 

 are the groups: 



>C = C< =CO >C = NH -CH = N- -N = N- 



-N = -nC -n( =n = o =c = s -C-S a -C- 



^0 x 



The structure of compounds is very intimately connected with their 

 color, and by means of color differences it is often possible to differentiate 

 isomers. An example of the latter case is the following: 



C 6 H 5 N-N ' S0 3 K C 9 H 5 N-N S0 8 K 



Syn-benzene-diazo-sulfonate, orange. Anti-benzene-diazo-sulfonate, yellow. 



The following are typical examples of the effect of chromophores: 



Colored. Colorless. 



C 8 H S N = N C 8 H 8 Azobenzene. C 8 H S NH NHC B H 5 Hydrazobenzene. 



C 5 H 3 N = Nitrosobenzene. C 8 H 6 N-H-OH Phenylhydroxylamine. 



= C 8 H 4 = Quinone. HO C 6 H 4 OH Hydroquinone. 



C.H. CO CO CO C 8 H 8 C H 5 CO CH a CO C 8 H 6 



Diphenyltriketone. Dibenzoylmethane. 



The introduction of a radical into an organic compound usually either 

 weakens the color or increases it. An interesting question comes up as to 

 whether color-changes are in any way related to energy changes in these 

 chemical reactions. A bathochrome causes the absorption bands to be 

 wider, while an auxochrome causes the intensity of the absorption to be 

 greater. A hypsochrome causes the absorption bands to narrow, a dimino- 



