Bond dissociation energy of CH3-H bond is 103 kcal mol-1 and heat of formation of CH4(g) as - 17.88 kcal mol-1. The heat of formation of methyl radical is

CH4(g)=CH3(g)+H( atom, g)ΔHfCH3−HCH3−H=103=ΔHfCH3,g+ΔHf(H atom ,g)−ΔHfCH4,gΔHfCH3,g=103−1032+(−17.889)=33.611kcalmole−1∴Heat of formation of methyl radical isΔH=33.611kcalmol−1