For the radical halogenation of methane, fluorine is the most reactive and iodine is the least reactive. The reason for this can be seen in the enthalpy for the first propagation step in the different halognations of methane. For fluorine the step is exothermic, however, for chlorine, bromine, and iodine the step is endothermic. This trend comes from the relative bond strengths of the H-X bond which is formed. The H-F bond is strong which causes the high reactivity of fluorine in these reactions. Correspondingly, reactions with fluorine have a relatively small activation energy and Iodine have relatively large activation energy. In the case of Iodine the energy of activation is so high that Iodination of methane does not occur.