The amount of energy needed to form the radicals CH3, 10 , 2o and 3o decreases in the order

Free radicals are electron deficient species, [due to they possess 7e-s]As the number of +IE groups [alkyl groups] increases,electron deficiency on the radical carbon decreases,i.e., easy of gaining of octet configuration increases,therefore stability of free radical increases.→ As the stability of free radical increases,its formation from its mother molecule increases,i.e., in isobutane the tertiary C – H is weaker and hence the 3o C–H bond breaking easily takes place and requires less energy [more reactive] and easily forms the 3o free radical.B.E order: