The Question is from neet-chemistry classification-of-elements-and-periodicity-in-prop spdf-block, and it was created by the author Mr. S M HUSSAIN

Question

Arrange the following ions in decreasing order of their ionic radii:  $Ce^{3+}$, $Pr^{3+}$, $Nd^{3+}$, $Pm^{3+}$

Mr. S M HUSSAIN · Accepted Answer

## CORE INFORMATION
✓ **Answer**: A
## CONCEPT FRAMEWORK
 **Primary Concept**: Lanthanide Contraction
 **Related Topics**: Ionic radii, atomic size, effective nuclear charge
 **Prerequisites**: Understanding of periodic trends and electronic configuration of lanthanides.
## SOLUTION PATHWAY
**Given Information**:
  Ions: $Ce^{3+}$, $Pr^{3+}$, $Nd^{3+}$, $Pm^{3+}$
  Task: Arrange in decreasing order of ionic radii.
**Method & Steps**:
 1. Identify the elements: These are all trivalent lanthanide ions.
  Reasoning: Lanthanides are elements in the f-block of the periodic table.
  Formula/Rule:  Across the lanthanide series, the ionic radii decrease due to the lanthanide contraction.
 2. Apply the lanthanide contraction principle.
  Working: The effective nuclear charge increases across the lanthanide series due to poor shielding of the 4f electrons.  This increase in effective nuclear charge pulls the electrons closer to the nucleus, resulting in a decrease in ionic radii.
  Key Point: The decrease in ionic radii is not linear, but the general trend is a decrease from left to right.
 3. Arrange the ions in decreasing order of ionic radii.
  Result: $Ce^{3+} > Pr^{3+} > Nd^{3+} > Pm^{3+}$
  Verification: This order follows the general trend of decreasing ionic radii across the lanthanide series.
 ## OPTION ANALYSIS
A ✓: $Ce^{3+} > Pr^{3+} > Nd^{3+} > Pm^{3+}$: Correctly reflects the decreasing ionic radii due to lanthanide contraction.
B :x: $Pm^{3+} > Nd^{3+} > Pr^{3+} > Ce^{3+}$: This is the reverse order, incorrect.
C :x: $Nd^{3+} > Pm^{3+} > Ce^{3+} > Pr^{3+}$: This order is incorrect and does not follow the lanthanide contraction trend.
D :x: $Pr^{3+} > Ce^{3+} > Pm^{3+} > Nd^{3+}$: This order is also incorrect and does not follow the lanthanide contraction trend.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Ignoring the lanthanide contraction: Students might incorrectly predict the trend based on general periodic trends without considering the specific effect of the 4f electrons.
   [How to avoid]: Remember that the poor shielding of 4f electrons leads to a greater effective nuclear charge and smaller ionic radii across the lanthanide series.
  2.Reversing the order:  Students might reverse the order of decreasing radii.
   [How to avoid]: Carefully consider the effect of increasing effective nuclear charge and its impact on ionic size.
 **Quick Tips**:
  1.Remember the lanthanide contraction mnemonic:  Across the lanthanides, radii shrink.
  2.Visualize the increasing nuclear charge pulling the electrons closer to the nucleus.

Mr. S M HUSSAIN · Answer

The correct Option is "$Ce^{3+} > Pr^{3+} > Nd^{3+} > Pm^{3+}$"

Mr. S M HUSSAIN · Answer

$Pm^{3+} > Nd^{3+} > Pr^{3+} > Ce^{3+}$

Mr. S M HUSSAIN · Answer

$Nd^{3+} > Pm^{3+} > Ce^{3+} > Pr^{3+}$

Mr. S M HUSSAIN · Answer

$Pr^{3+} > Ce^{3+} > Pm^{3+} > Nd^{3+}$

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