Final answer:
Extra electrons in an anion reside in the outer energy levels of the anion, as per the Aufbau principle. This leads to an increase in the electron cloud size, making the anion larger than its parent atom. The specific order in which these levels are filled or emptied depends on whether the element is a main group element or a transition metal.
Step-by-step explanation:
The extra electrons in an anion go to the outer energy levels of the anion. Since anions form when atoms gain electrons, these extra electrons increase the size of the electron cloud around the nucleus. According to the Aufbau principle, electrons fill the atomic orbitals of the lowest available energy levels before occupying higher levels. Thus, in the case of transition metals and inner transition metals, the added electrons fill in the order of the principle, which might be different from main group elements.
For instance, the s orbitals are typically filled before the d or f orbitals. When the electron configurations of ions are considered, for main group elements, electrons added last are the first removed when forming cations. For transition metals, rather than losing d or f electrons, the highest ns electrons are usually removed first to form cations. Conversely, when these atoms gain electrons and form anions, the extra electrons occupy the outermost energy levels, leading to a larger ion compared to the parent atom due to increased electron-electron repulsion and weaker overall attraction by the protons.