Which of the following is not iso electronic with H_2 ~S ?

Key Idea Iso electronic species are those which have same number of electrons. The total number of electrons in the given species are as follows $$\begin{aligned} \mathrm{H}_2 \mathrm{ S} & =2+16=18 \text{ electrons } \\ \mathrm{F}_2 & =9+9=18 \text{ electrons } \\ \mathrm{O}^{2-} & =8+2=10 \text{ electrons } \\ \mathrm{Ca}^{2+} & =20-2=18 \text{ electrons } \\ \mathrm{Sc}^{3+} & =21-3=18 \text{ electrons } \end{aligned}$$ Thus, $\mathrm{O}^{2-}$ (oxide ion) is not iso electronic with $\mathrm{H}$_2 $\mathrm{ S} .$