Isoelectronic Series Radius

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In isoelectronic series O2−, F−, Na+, Mg2+, Al3+ ionic radius order (largest to smallest):

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1. **Isoelectronic Definition**: Species that have the same number of electrons (in this case, $10$ electrons, with the $Ne$ configuration).
2. **Nuclear Charge Control**: Since electron-electron repulsion is roughly the same across the series, the size is determined solely by the number of protons ($Z$).
3. **Z values**:
   - $O^{2-}$: $Z=8$
   - $F^-$: $Z=9$
   - $Na^+$: $Z=11$
   - $Mg^{2+}$: $Z=12$
   - $Al^{3+}$: $Z=13$
4. **Inverse relationship**: Higher nuclear charge pulls the $10$ electrons more tightly. Thus, **higher $Z \implies$ smaller radius**.
5. **Order**: **$O^{2-} > F^- > Na^+ > Mg^{2+} > Al^{3+}$**.

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Isoelectronic Series Radius

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