American Chemical Society (ACS) Chemistry Practice Exam

Anions are larger than cations due to the differences in their electronic structures and the effects of nuclear charge. Anions, which are negatively charged, gain one or more electrons. This addition of electrons increases electron-electron repulsion within the electron cloud, leading to a greater distance between the electrons and, consequently, an increase in the size of the electron cloud. On the other hand, cations are positively charged and lose one or more electrons. The loss of electrons results in reduced electron-electron repulsion and a tighter pull on the remaining electrons by the nucleus, often leading to a smaller ionic radius compared to the neutral atom from which they came. Neutral atoms fall between anions and cations in size. They typically have a balanced number of protons and electrons, resulting in a specific radius that is smaller than that of anions but larger than that of cations. All ions are certainly not the same size, as their radii depend significantly on their charge and the number of electrons present, making it clear why anions are generally larger than both cations and neutral atoms.