Among the species ${\mathrm{Ce}}^{4+},{\mathrm{Eu}}^{2+},{\mathrm{Tb}}^{4+},{\mathrm{Yb}}^{2+},{\mathrm{Sm}}^{2+}, {\mathrm{Tm}}^{2+},{\mathrm{Pr}}^{4+},{\mathrm{Nd}}^{4+}$, find the number of ions which have sufficient stability to be studied in aqueous chemistry.

Low ionization energy , high electron affinity and stable electronic configuration  of the element-forming cation is the first need for stable ionic bond formation.

The stable ions are,

$\begin{array}{l}{\mathrm{Ce}}^{4+}⟶\left(f^0\right)\\ {\mathrm{Eu}}^{2+},{\mathrm{Tb}}^{4+}⟶\left(f^7\right)\\ {\mathrm{Yb}}^{2+}⟶\left(f^{14}\right)\end{array}$

$\begin{array}{r}{\mathrm{Sm}}^{2+}⟶\left(f^6\right)\end{array}$(similar to $\begin{array}{r}{\mathrm{Eu}}^{2+}\end{array}$)

$\begin{array}{l}\\ {\mathrm{Tm}}^{2+}⟶\left(f^{13}\right)\\ {\mathrm{Pr}}^{4+}⟶\left(f^1\right)\\ {\mathrm{Nd}}^{4+}⟶\left(f^2\right)\end{array}$

Hence, the number of ions which have sufficient stability to be studied in aqueous chemistry is 3.