Ions formed when atoms gain or lose electrons. A cation (positively charged ion) forms when one or more electrons removed from a parent atom. For main group elements, the electrons that 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 added to a parent atom. The added electrons fill in the order predicted by the Aufbau principle.