The simplest cation in which the cationic center is adjacent to a pi-system. Two resonance forms can be written that place the positive charge at either one of the terminal methylene carbon atoms. It is for this bonding situation that the allyl cation is symmetric with equal amounts of charge on both terminal C-atoms. Compare the bonding situations of allyl cation and allyl radical. Do you expect the C-C bond length to be longer or shorter in allyl cation compared to allyl radical?