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Isoelectronic species have the same number of electrons. Properties vary only with nuclear charge.

Common 10-electron series: N^3-, O^2-, F^-, Ne, Na^+, Mg^2+, Al^3+. Radius: N^3- > O^2- > F^- > Ne > Na^+ > Mg^2+ > Al^3+ (more Z = smaller). IE: N^3- < O^2- < F^- < Ne < Na^+ < Mg^2+ < Al^3+ (more Z = harder to remove electron). Hydration energy: Al^3+ > Mg^2+ > Na^+ (higher charge density = stronger hydration).

Common 18-electron series: P^3-, S^2-, Cl^-, Ar, K^+, Ca^2+. Same principles apply: more protons = smaller radius.

JEE strategy: When asked to arrange isoelectronic species, first confirm they have the same electron count. Then rank by nuclear charge — higher Z = smaller radius, higher IE. This is one of the simplest yet most frequently tested concepts.