Well...if it was an alkyl group on a C chain, whether it broke off homolytically or heterolytically. If it was homolytically, one electron from the bond would go with the methyl group, and the other with the chain - forming free radicals. If it was heterolytically, either the bonding electrons would go with the methyl group, forming a carbANion (negatively charged C), and the carbon chain would have a carbocation, or the bonding electrons would stay on the chain, forming a methyl group with a carbocation, etc
If it was originally a methane (CH4), it would depend how that H atom came off of the molecule, ie. whether a nucleophile came in and plucked the H off, which would leave a carbANion, or whether somehow the bonding electrons came with it (electrophilic reaction), forming a carboCATion.
I'd read the free radical section of your textbook if I were you. I need to as well.