the reactivity of the halogens with #CH_4# decreases with in the group due the less electronegativity
#CH_4+4F_2=> CF_4+4HF## at low temperature# #CH_4+4Cl_2=>CCl_4+4HCl## at slightly high temperature# #CH_4+4Br_2=>CBr_4+4HBr##(at moderatly high temperature)# #CH_4+4I_2<=>_(catalyst)CI_4+4HI#(even at high temperature)#