Q.44 Among K+, Li+, Rb+, Cs+, the number of cation(s) having ionic radii more than Na+
is/are _______.
Among K⁺, Li⁺, Rb⁺, Cs⁺, four cations have ionic radii larger than Na⁺.
Ionic radii increase down Group 1 of the periodic table due to additional electron shells with poor shielding, reducing effective nuclear charge per electron. Standard Shannon effective ionic radii (6-coordinate) confirm this trend for these monovalent cations.
Ionic Radii Comparison
| Cation | Ionic Radius (pm, CN=6) | Larger than Na⁺ (116 pm)? |
|---|---|---|
| Li⁺ | 90 | No |
| Na⁺ | 116 | Reference |
| K⁺ | 152 | Yes |
| Rb⁺ | 166 | Yes |
| Cs⁺ | 181 | Yes |
Li⁺ is smaller than Na⁺ as it has fewer shells despite similar charge. K⁺, Rb⁺, and Cs⁺ exceed Na⁺ due to progressively larger shells.
Detailed Option Analysis
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Li⁺: Smallest ionic radius (90 pm) from highest effective nuclear charge pulling electrons closer. Not larger than Na⁺.
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K⁺: Third shell added; radius jumps to 152 pm, exceeding Na⁺. Larger.
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Rb⁺: Fourth shell; 166 pm radius confirms increase down the group. Larger.
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Cs⁺: Sixth shell; largest at 181 pm. Larger.
Thus, K⁺, Rb⁺, Cs⁺ (3 cations) have ionic radii more than Na⁺; Li⁺ does not. Answer: 3.
Introduction to Ionic Radii K+ Li+ Rb+ Cs+ Na+
Ionic radii K+ Li+ Rb+ Cs+ Na+ follow a clear periodic trend vital for exams like CSIR NET Life Sciences. These alkali metal cations (Group 1) lose one electron to form +1 ions, with sizes increasing down the group due to added shells. Question: Among K+, Li+, Rb+, Cs+, how many exceed Na+ (116 pm)? This guide breaks it down with data.
Periodic Trend Explanation
Down Group 1, valence electrons occupy new shells farther from the nucleus, expanding ionic radii despite constant +1 charge. Effective nuclear charge rises slightly but shielding dominates, yielding: Li⁺ (90 pm) < Na⁺ (116 pm) < K⁺ (152 pm) < Rb⁺ (166 pm) < Cs⁺ (181 pm). Coordination number (CN=6) standardizes comparisons via Shannon-Prewitt values.
Shannon Ionic Radii Data Table
| Ion | CN | Radius (pm) | vs Na⁺ (116 pm) |
|---|---|---|---|
| Li⁺ | 6 | 90 | Smaller |
| Na⁺ | 6 | 116 | Baseline |
| K⁺ | 6 | 152 | +36 pm |
| Rb⁺ | 6 | 166 | +50 pm |
| Cs⁺ | 6 | 181 | +65 pm |
Data from reliable sources confirms K+, Rb+, Cs+ surpass Na+.
CSIR NET Exam Application
In CSIR NET questions like “number of cations with ionic radii more than Na+”, select 3 (K⁺, Rb⁺, Cs⁺). Li⁺ fails due to no extra shell beyond He-core. Use for crystal structures, hydration, or lattice energy problems.
