SOME PHOTOCHEMICAL CON8IDER.ATIONS 17 



endothermic formation of ozone needs 68000 cal per 3 mole O2, the efficiency 

 is 



68000 



V = 



137000 



= 50% 



§ 8 The Nature of the Free Radicals 



Many investigators believe that free radicals have important functions in 

 photosynthesis. Free radicals are atoms or atom groups possessing unpaired 

 electrons in their electronic structures. Using G. N. Lewis' method of writ- 

 ing chemical formulae, each valency electron of an element is indicated by a 

 dot so that the usual bars between the symbols are replaced by two dots 

 representing an electron pair. The formula for ammonia shows that the 

 five valency electrons of the N atom are combined with the three valency 

 electrons of the three H atoms resulting in four electron pairs 



:N: + 3 H- -* H:N:H 



H 



In this formula one of the electron pairs does not serve as a link between two 

 atoms. Both electrons of this so-called lone electron pair belong to the N 

 atom. The formula for the ammonium ion shows the same number of elec- 

 tron pairs 



H 

 H:N + :H 



H 



As only four and not hve electrons belong to the N atom, there is an excess 

 of one positive elementary charge characterized by a plus sign. 



Coming back to the definition of the free radical, we see that the H atom and 

 the halogen atoms with seven valency electrons must be considered to be free 

 radicals 



H« :C1. li- 



the unpaired electron being represented by a bold dot. 



A free radical with one unpaired electron is called monoradical, whereas 

 a free radical with two unpaired electrons is called a biradical. Thus, the H 

 atom and the halogen atoms are free monoradicals. They are very active 

 substances formed when halogen acids are photochemically dissociated. The 

 primary photochemical reaction described in § 6 can be written as follows 



lip/it 

 H:I: > H. + •!: 



Other examples of free monoradicals are HO • and the still somewhat prob- 

 lematic H02* 



