Trend in Radical Stability
3o > 2o > 1o > Methyl
In order to understand the reason for this ordering in radical stability we must first look at the structure of a alkyl radical.
Experimental data have shown that radicals have a trigonal planar geometry associated with sp2 hybridization.
The unpaired electron is contained in a p orbital which is perpendicular to the molecular plane.
This trigonal planar geometry allows for sigma on adjacent carbons to align and overlap with the p orbital. The alignment allows for the electrons in the sigma bond to donate electrons into the electron deficient p orbital. This process is called hyperconjugation.
The more alkyl substituents, and therefore more sigma bonds, the electron density is donated into the p orbitals. This is why the general trend in radical stability is true 3o > 2o > 1o > Methyl.