...and in fact it is the bromine molecule, with a WEAKER #Br-Br# bond, that undergoes photolysis to give TWO neutral bromine radicals...
#Br_2stackrel(hnu)rarr2dotBr#
#dotBr+H_2CR_2 rarr Br-H + dotCHR_2#
#Br_2 + dotCHRrarrdotBr+BrCHR_2#
You can look up #"initiation, propagation, termination steps"# of radical reactions in your text; this is worth doing. It is the propagation step that is the important step. The formation of a radical, and its subsequent reaction, creates ANOTHER radical, which can propagate the radical chain of reaction....
