We have learned that radicals can be formed through the homolytic cleavage of two-electron single bonds, or via the "flow" of a pre-existing radical to another position. There is a third process that forms radicals: single-electron transfer (SET). An electron-rich species willing to part with an electron can give one away to an electron-poor molecule. If both molecules begin with an even number of electrons, the transfer of one electron makes both products odd-electron species (radicals). Furthermore, since the electron is negatively charged, the "giver" becomes positively charged and the "receiver" becomes negatively charged. Put another way, a radical cation and radical anion result from single-electron transfer between neutral molecules