Heterolytic bond cleavage is a process where the electron pair that comprised a bond moves to one of the atoms that was formerly joined by a bond.  The bond breaks, forming a negatively charged species (an anion) and a positively charged species (a cation).  The anion is the species that retains the electrons from the bond while the cation is stripped of the electrons from the bond. The anion usually forms on the most electronegative atom, in this example atom A. This is because the most electronegative atom will naturally attract electrons towards itself more strongly, leading to its negative charge.