Chemically, this result is simple to comprehend. When ionizing the parent, we remove electrons from the valence shell, which is responsible for the chemical bonds holding the molecule together. Thus, if too many electrons are removed, fragmentation will be thermodynamically preferable. Note that the parent species still might exist, but as a metastable state, protected by the activation energy for the fragmentation reaction 1. The charge at which the parent molecule will become thermodynamically unstable for fragmentation into fragments "X" and "Y" is denoted as the maximum charge Q XY,max , which can be obtained from Equation 10 by setting D XY (Q) = 0, giving