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)#