Read Isoelectronic Series Before Ranking Any Radius
Radius comparisons reduce to one question every time: is nuclear charge changing while electron count stays fixed (an isoelectronic series, where more protons always wins), or is electron count itself changing (more electrons removed always shrinks the species)?
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Isoelectronic series: more protons, smaller radius
H⁻, Li⁺, Mg²⁺, Al³⁺ all have different electron counts and aren't isoelectronic with each other — but within a genuine isoelectronic set (same electron count, e.g. O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺, all 10 electrons), radius shrinks as nuclear charge rises: more protons pull the same electron cloud in tighter. Rb⁺, Sr²⁺, Br⁻, Se²⁻ is a similar isoelectronic-style comparison — rank by nuclear charge, not by which is a cation or anion.
Same element, more positive charge, smaller radius
Ti²⁺ > Ti³⁺ > Ti⁴⁺ — removing successive electrons from the SAME element always shrinks the ion further, since nuclear charge stays fixed while electron-electron repulsion drops with each removed electron. This same logic ranks Cr's ionic radius as smallest in its highest-oxidation-state compound (K₂CrO₄, Cr₆⁺) among a set of Cr compounds at different oxidation states.
Transition series show near-constant radius across several elements
Fe, Co, Ni show nearly identical atomic radii — as d-electrons fill across a transition series, the shielding they provide roughly cancels the increasing nuclear charge, flattening the radius trend compared to the sharp fall seen across a main-group period. This 'nearly the same radius' pattern is specific to consecutive transition metals, not a general periodic-table feature.
Coordination number and hypervalency both trace back to size
Aluminium forms AlF6³⁻ (coordination number 6) while boron forms only BF₄⁻ (coordination number 4) — boron is simply too small to accommodate six fluorines around it, despite both being group 13. Radius (and the resulting steric/orbital-availability limits) directly governs which coordination numbers and hypervalent species are geometrically and electronically possible.
OlympIQ exam lens
- ✓Within an isoelectronic series (same electron count), radius shrinks as nuclear charge (atomic number) rises.
- ✓Removing successive electrons from the same element always shrinks the resulting ion further (Ti²⁺ > Ti³⁺ > Ti₄⁺).
- ✓Consecutive transition metals (Fe, Co, Ni) show near-constant atomic radius, unlike the sharp fall across a main-group period.
- ✓Smaller central atoms (like B vs Al) support lower maximum coordination numbers — this is a size effect, not purely an electronic one.
Don't rank isoelectronic species by charge sign (cation vs anion) — rank strictly by nuclear charge (atomic number); a 2+ cation and a 2- anion with the same electron count are compared the same way as any other pair in that isoelectronic series.
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