Ionic radius is determined by electron configuration and effective nuclear charge, not by the neutral atom's periodic trends: when an atom ionizes, gaining or losing valence electrons changes its shell occupancy and thus its size independent of its proton count. Cations shrink because losing the outermost shell's electron(s) leaves a smaller, more tightly bound electron configuration (isoelectronic with the preceding noble gas), while anions expand because gaining electron(s) completes an additional shell without a compensating increase in nuclear charge. This is a core concept in periodic trends and ionic bonding, illustrating that atomic/ionic radius depends jointly on nuclear charge and electron shell structure, and that trends for neutral atoms cannot be extrapolated directly to their ions.
Ionic radius is determined by electron configuration and effective nuclear charge, not by the neutral atom's periodic trends: when an atom ionizes, gaining or losing valence electrons changes its she…