JEE Chemistry 2024 Question with Solution

Given: We need to identify which lanthanoid ions are diamagnetic.

Find: The pair of ions having no unpaired electrons.

Diamagnetic species have all electrons paired.

For the given ions:

• La$^{3+}$ has configuration $[\mathrm{Xe}]$, so it has no unpaired electrons.
• Ce$^{4+}$ has configuration $[\mathrm{Xe}]$ or equivalently $4f^0$ beyond the xenon core, so it also has no unpaired electrons.
• Nd$^{3+}$ and Eu$^{3+}$ contain unpaired $4f$ electrons, so they are paramagnetic.
• Lu$^{3+}$ is diamagnetic, but the option paired with Eu$^{3+}$ is not fully diamagnetic.

Therefore, the diamagnetic lanthanoid ions are La$^{3+}$ and Ce$^{4+}$. Hence, the correct option is B.

The solution contains a discrepancy where one heading says the correct option is C, but the working and conclusion clearly identify La$^{3+}$ and Ce$^{4+}$, which corresponds to B in the given options.

Given: Diamagnetic species must have all electrons paired.

Find: Which listed pair has both ions diamagnetic.

Check the electron configurations mentioned in the solution:

1. La$^{3+}$: $[\mathrm{Xe}]$
2. Ce$^{4+}$: $[\mathrm{Xe}]$ or $[\mathrm{Xe}]4f^0$

Both of these have no unpaired electrons, so both are diamagnetic.

Now compare with the remaining ions:

• Nd$^{3+}$ has unpaired $4f$ electrons.
• Eu$^{3+}$ also has unpaired $4f$ electrons.
• Lu$^{3+}$ is diamagnetic, but it is paired with Eu$^{3+}$ in the option list, so that option is not correct.

Thus, the only correct pair is La$^{3+}$ and Ce$^{4+}$, so the correct option is B.