The reactivity of hydrogen atoms attached to carbon atom in the halogenation of an alkane has the order :

Differences in reactivity can only be attributed to differences in C−H bond dissociation energies. The weaker bonds are broken more easily than the strong bonds. The difference in C−H bond dissociation energy reported for primary(1º)>secondary(2º)>tertiary(3º) sites agrees with the halogenation observations. 
​ It is also due to radical formed by the reactive hydrogen elimination, which is stabilized by hyperconjugation with adjacent hydrogen atoms.