$word_{count}$: 180

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.