Plenty of atoms can be ionized to -1 charge. (I'm not sure if this is true of all of them. E.g. perhaps Helium and other noble gases cannot be; also perhaps Zinc, Magnesium, Beryllium, Nitrogen, Manganese are immune.) But no atom can be ionized to -2 charge -- i.e. no such ion is stable in isolation. There is, in fact, a naive approximation, called Thomas-Fermi-von Weizsacker, to quantum mechanics in which it is a theorem (by Benguria & Lieb) that the max possible negative charge you can put on an N-atom molecule, N>0, is at most N extra electrons. I would conjecture that asymptotically, the most negative charge you can put on an N-atom molecule is going to be constant*sqrt(N). This would be achieved for a hollow metal sphere, regarded as a giant "molecule," with thickness of order 1 atoms, radius r where N is of order r^2; charging to order sqrt(N) would give it a constant electric field at surface. But this conjecture is completely open. QUESTION: what is the most negative charge f(N) that can be put (stably) on an N-atom molecule for small values of N? E.g. sulfite (SO3) or carbonate (CO3) each with -2 charge both show f(4)>=2, assuming this ion actually is stable in isolation in vacuum. Arsenite AsO3 with charge -3 shows f(4)>=3 assuming same. The AsS (charge -3) and allylenide [C=C=C] with charge -4 ions each would be enough to refute that Thomas-Fermi-von Weisacker theorem for actual quantum mechanics, if these ions can exist stably in isolation (which I doubt, but they exist inside some ionic crystals). -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)