Chemistry · PYQ NEET UG 2021

From the following pairs of ions which one is not an isoelectronic pair?

1. \( \text{Na}^+, \text{Mg}^{2+} \)
2. \( \text{Mn}^{2+}, \text{Fe}^{3+} \)
3. \( \text{Fe}^{2+}, \text{Mn}^{2+} \)
4. \( \text{O}^{2-}, \text{F}^- \)