Carbocation stability :

$\stackrel{+}{\text{C}}{\text{H}}_{\text{3}}{\text{<CH}}_{\text{3}}\stackrel{+}{\text{C}}{\text{H}}_{\text{2}}{\text{<(CH}}_{\text{3}}{\text{)}}_{\text{2}}\stackrel{+}{\text{C}}{\text{H<(CH}}_{\text{3}}{\text{)}}_{\text{3}}\stackrel{+}{\text{C}}\text{ ::}$ alkyl radical stability

The observed order of carbocation stability is $\stackrel{+}{\mathrm{C}}{\mathrm{H}}_3<{\mathrm{CH}}_3\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2<{\left({\mathrm{CH}}_3\right)}_2\stackrel{+}{\mathrm{C}}\mathrm{H}<{\left({\mathrm{CH}}_3\right)}_3\stackrel{+}{\mathrm{C}}$

The stability of carbocation and free radicals increases in a similar manner as the number of electron-donating groups(-CH₃) directly linked to carbon atom, containing radical or cation increases. Thus, stability of alkyl radical increases as we proceed from primary to tertiary alkyl free radical.