The simplest radical in which a radical center is adjacent to a pi-system. Two resonance forms can be written that place the radical at either one of the terminal methylene carbon atoms. It is for this bonding situation that the allyl radical is symmetric with equal amounts of spin density on both terminal C-atoms. (This electronic structure is very different from what one would call a vinyl substituted methyl radical.) The C-C bond length in allyl radical is 137.7 pm.