Question: The correct order of decreasing radius is :-...

Question

The correct order of decreasing radius is :-

Answer 1

Solution

The given elements are Lanthanum (La), Cerium (Ce), Europium (Eu), and Ytterbium (Yb). These are elements from the lanthanide series. The atomic numbers are:

La: 57 Ce: 58 Eu: 63 Yb: 70

Across the lanthanide series (from La to Lu), there is a gradual decrease in the atomic and ionic radii with increasing atomic number. This phenomenon is known as lanthanide contraction. It is caused by the poor shielding effect of the 4f electrons, which leads to an increase in the effective nuclear charge experienced by the outer electrons, pulling them closer to the nucleus.

The question asks for the correct order of decreasing radius. While the question does not specify whether it refers to atomic or ionic radii, questions concerning the trend across the lanthanides usually refer to the ionic radii of the most common oxidation state, which is +3. The lanthanide contraction is more pronounced and regular for the ionic radii of the M $^{3+}$ ions compared to the atomic radii, where anomalies exist for elements like Eu and Yb due to their tendency to exhibit a +2 oxidation state in the metallic state.

Let's consider the ionic radii of the M $^{3+}$ ions:

La $^{3+}$ : 103 pm (for coordination number 6) Ce $^{3+}$ : 101 pm (for coordination number 6) Eu $^{3+}$ : 95 pm (for coordination number 6) Yb $^{3+}$ : 86 pm (for coordination number 6)

Based on these values, the order of decreasing ionic radius is:

La $^{3+}$ > Ce $^{3+}$ > Eu $^{3+}$ > Yb $^{3+}$

Translating this order back to the elements, the order of decreasing radius is:

La > Ce > Eu > Yb

This order follows the expected trend of decreasing radius with increasing atomic number (Z=57 < Z=58 < Z=63 < Z=70) due to lanthanide contraction.