Ionic Radius

An atom forms a positively-charged ion (a CATION) when it loses one or more electrons, and a negatively-charged ion (an ANION) when it gains one or more electrons. Ionic radii are estimated by measuring the distance between neighbouring cations and anions packed together in an ionic crystal, and they show exactly the same across-period and down-group trends as ordinary atomic radii (section 7.5.2.1). A cation is always smaller than the neutral atom it came from: it has fewer electrons than the parent atom while carrying the same nuclear charge, so the reduced electron-electron repulsion (weaker shielding) leaves a larger effective nuclear charge acting on the remaining electrons, pulling them in tighter. An anion, conversely, is always larger than its parent atom: it has MORE electrons than the neutral atom, so the extra electron-electron repulsion lowers the effective nuclear charge felt by each electron, letting the electron cloud expand. A special case arises with ISOELECTRONIC SPECIES — different atoms or ions that happen to share exactly the same total electron count but have different actual nuclear charges. Among such species, the one with the LARGER nuclear charge attracts its (identical-sized) electron cloud more strongly and t …