JEE Chemistry 2025 Question with Solution

Given: Lanthanoid ions are listed, and we need to identify which have the configuration $4f^7$.

Find: The correct option containing the ions with $4f^7$ configuration.

Write the electronic configurations given in the solution:

$$\text{Eu}^{2+} : [\text{Xe}]\,4f^7\,6s^0$$ $$\text{Gd}^{3+} : [\text{Xe}]\,4f^7\,5d^0\,6s^0$$ $$\text{Eu}^{3+} : [\text{Xe}]\,4f^6\,6s^0$$ $$\text{Tb}^{3+} : [\text{Xe}]\,4f^8\,6s^0$$ $$\text{Sm}^{2+} : [\text{Xe}]\,4f^6\,6s^0$$

Only Eu$^{2+}$ and Gd$^{3+}$ have the configuration $4f^7$.

Therefore, the correct pair is (A) and (B) only.

The correct option is A.

Given: We must determine the $4f$ electron count for each ion.

Find: Which pair among the listed ions has configuration $4f^7$.

Recall the general lanthanoid configuration:

$$[\text{Xe}]\,4f^{1-14}\,5d^{0-1}\,6s^2$$

Now evaluate each species:

• Eu$^{2+}$: neutral Eu is $[\text{Xe}]\,4f^7\,6s^2$, so after losing two electrons, Eu$^{2+}$ becomes $[\text{Xe}]\,4f^7$.
• Gd$^{3+}$: neutral Gd is $[\text{Xe}]\,4f^7\,5d^1\,6s^2$, so after losing three electrons, Gd$^{3+}$ becomes $[\text{Xe}]\,4f^7$.
• Eu$^{3+}$ becomes $[\text{Xe}]\,4f^6$.
• Tb$^{3+}$ becomes $[\text{Xe}]\,4f^8$.
• Sm$^{2+}$ becomes $[\text{Xe}]\,4f^6$.

Thus, only Eu$^{2+}$ and Gd$^{3+}$ satisfy the condition.

Therefore, the correct option is A.