The ions O²⁻, F⁻, Na⁺, Mg²⁺, and Al³⁺ are isoelectronic. Their ionic radii show:

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1.  **Isoelectronic Species:** The ions $O^{2-}$, $F^-$, $Na^+$, $Mg^{2+}$, and $Al^{3+}$ all have the same number of electrons (10 electrons) but different nuclear charges (protons: O=8, F=9, Na=11, Mg=12, Al=13).
2.  **Effect of Nuclear Charge:** For isoelectronic species, the ionic radius is determined by the nuclear charge. As the atomic number (nuclear charge) increases, the attraction between the nucleus and the same number of electrons increases.
3.  **Trend:** A higher nuclear charge pulls the electron cloud closer to the nucleus, resulting in a smaller radius. Therefore, as we move from $O^{2-}$ to $Al^{3+}$, the nuclear charge increases, causing a **significant decrease** in ionic radius (Source 85 confirms that radius generally decreases with increasing nuclear charge).

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