The ions O2$-$, F$-$, Na+, Mg2+ and Al3+ are isoelectronic. Their ionic radii show
a significant increase from O2$-$ to Al3+
a significant decrease from O2$-$ to Al3+
an increase from O2$-$ to F$-$ and then decrease from Na+ to Al3+
a decrease from O2$-$ to F$-$ and then increase from Na+ to Al3+.
