Final answer:
Extra electrons in an anion are added to the s and p orbitals of the outermost energy level based on the Aufbau principle, often resulting in a noble gas configuration and increasing the size of the electron cloud.
Step-by-step explanation:
The extra electrons in an anion are added to a parent atom's electron cloud. According to the Aufbau principle, these electrons fill the atom's orbitals starting with the lowest energy levels. Typically, the added electrons enter the s and p orbitals of the outermost energy level to achieve a noble gas configuration.
For example, the oxygen atom, with the electron configuration 1s²2s²2p⁴, gains two electrons to become an oxide anion (O²⁻) with the electron configuration of neon (Ne), 1s²2s²2p⁶. This addition of electrons causes the anion to be larger than the parent atom and results in increased electron-electron repulsion.
When working with covalent structures, leftover electrons that do not form bonds are placed as lone pairs on the atoms, usually fulfilling the octet rule or achieving a stable electron configuration.