Question

Arrange the following ions in increasing order of ionic radii:
(A) Eu$^{3+}$
(B) Lu$^{3+}$
(C) Y$^{3+}$
(D) La$^{3+}$
Choose the correct answer from the options given below:

• A. A $<$ B $<$ C $<$ D
• B. A $<$ C $<$ B $<$ D
• C. B $<$ C $<$ A $<$ D
• D. C $<$ B $<$ D $<$ A

Answer 1

The Correct Option is D

The ionic radii generally increase down a group in the periodic table. However, for the
lanthanide series, there are some considerations:
1. \(Y^{3+}\) : It has the smallest ionic radius among these ions due to its position in the periodic
table and the high effective nuclear charge.
2. \(Lu^{3+}\) : Although it is at the end of the lanthanide series, the ionic radius is relatively
small due to the increase in nuclear charge.
3. \(La^{3+}\) : This ion has a larger radius than Lu3+ and Y3+ because it is located above in the
lanthanide series.
4. \(Eu^{3+}\) : This ion has the largest ionic radius among the listed ions.
Therefore, the correct increasing order of ionic radii is:

\(Y^{3+} < Lu^{3+} < La^{3+} < Eu^{3+}\)

Answer 2

The ionic radii generally increase down a group in the periodic table. However, for the
lanthanide series, there are some considerations:

• \(Y^{3+}\): It has the smallest ionic radius among these ions due to its position in the periodic
  table and the high effective nuclear charge. As a result, it has a relatively stronger pull on its electrons, leading to a smaller size.
• \(Lu^{3+}\): Although it is at the end of the lanthanide series, the ionic radius is relatively
  small due to the increase in nuclear charge. The increase in nuclear charge outweighs the shielding effect, leading to a smaller radius.
• \(La^{3+}\): This ion has a larger radius than \(Lu^{3+}\) and \(Y^{3+}\) because it is located above in the
  lanthanide series. The additional electrons in the inner shell result in a greater shielding effect, making the radius larger than expected for its position.
• \(Eu^{3+}\): This ion has the largest ionic radius among the listed ions. Its relatively larger radius is attributed to the electron configuration and the effects of the lanthanide contraction, which causes less effective nuclear charge attraction in comparison to other ions.

It’s important to note that the lanthanide series exhibits a phenomenon called the lanthanide contraction, where the ionic radii decrease slightly with increasing atomic number, despite the expectation of an increase in size due to the filling of the 4f orbitals. This is a result of poor shielding by the 4f electrons, leading to a greater nuclear pull on the outer electrons.

Therefore, the correct increasing order of ionic radii is:

\(Y^{3+} < Lu^{3+} < La^{3+} < Eu^{3+}\)