Final answer:
The atomic radius of ions changes when atoms become cations or anions due to electron loss or gain, which alters electron-electron repulsions and effective nuclear charge. Cations are smaller than their parent atoms, while anions are larger. Additionally, cations with larger charges are smaller than those with smaller charges.
Step-by-step explanation:
The atomic radius of an ion changes when an atom becomes a cation or an anion. A cation, having lost one or more electrons, is always smaller than the parent atom. This decrease in size is because the loss of electrons reduces electron-electron repulsions in the valence shell and increases the effective nuclear charge on the remaining electrons, pulling them closer to the nucleus. For instance, an aluminum atom has a covalent radius of 118 pm, but as a cation (Al³⁺), the ionic radius is only 68 pm.
Conversely, an anion is always larger than the parent atom because it gains electrons, which increases repulsion among the electrons in the same shell. Additionally, the extra electrons increase the electrostatic shielding effect and decrease the effective nuclear charge on the outermost electrons, making the radius larger. For example, a sulfur atom will have a larger radius as an anion (S²⁻) than as a neutral atom.
Moreover, trends show that cations with larger charges are smaller than those with smaller charges. For example, a V²⁺ cation has a radius of 79 pm, whereas a V³⁺ cation is just 64 pm. When moving down a group in the periodic table, the radii of cations with the same charge tend to increase due to a rise in the principal quantum number, n.