Free radicals are such chemical species which have an odd electron pair or unpaired electron in the p orbital. Due to unpaired electron the radical species are paramagnetic in nature.
We rank radicals in order of stability mostly on the basis of two main factors. They are:
Stability by resonance
Stability is achieved by adjacent atoms with lone pairs.
However, the increase in stability depends on the electronegativity of the atom.
So, basically, radicals will have higher stability as the atom’s electronegativity decreases.
Thus, stability increases in the order methyl < primary < secondary < tertiary.
Some of the examples of formation of radical species given below:
