electrons
Electron Configurations of Ions We have seen that ions are formed when atoms gain or lose electrons. A cation positively charged ion forms when one or more electrons are removed from a parent atom. For main group elements, the electrons that were added last are the first electrons removed. For transition metals and inner transition metals, however, electrons in the s orbital are easier to remove than the d or f electrons, and so the highest ns electrons are lost, and then the n 1 d or n 2 f electrons are removed. An anion negatively charged ion forms when one or more electrons are added to a parent atom. The added electrons fill in the order predicted by the Aufbau principle.