Bohr Radius
The radius of the first Bohr orbit for hydrogen is 0.529 Å. What would be the radius of the second orbit for Li^{2+} ion (Z=3)?
Select the correct option:
Solution
0.706 Å
The radius of the nth Bohr orbit for a hydrogen-like ion is given by r_n = a_0 × n^2 / Z, where a_0 = 0.529 Å is the first Bohr radius for hydrogen, n is the principal quantum number, and Z is the atomic number of the ion. This formula captures two competing effects: higher n increases the orbital radius while higher Z contracts it due to stronger nuclear attraction. For Li^{2+} with Z=3 and n=2: r_2 = 0.529 × (2)^2 / 3 = 0.529 × 4 / 3 = 2.116 / 3 = 0.705 Å ≈ 0.706 Å. Option 0.353 Å would correspond to r_1 of Li^{2+} (n=1): 0.529 × 1/3 = 0.176 Å — actually this value (0.353 Å) arises if n=1 and Z=1.5, which is not physical. Option 0.529 Å is the first Bohr radius for hydrogen (n=1, Z=1) and does not apply here. Option 1.058 Å would arise from using Z=1 (hydrogen) with n=2: 0.529 × 4/1 = 2.116 Å — or it could represent a factor-of-2 error. This problem is a direct application of Bohr's orbit radius formula extended to hydrogen-like ions, a core NCERT and JEE topic. Plausibility check: for Li^{2+} (Z=3) at n=2, the radius is 4/3 × 0.529 ≈ 0.706 Å, which is smaller than the hydrogen n=2 radius (4 × 0.529 = 2.116 Å) due to the higher nuclear charge, confirming the answer is physically reasonable.
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About This Question
- Subject
- chemistry
- Chapter
- atomic structure
- Topic
- bohr radius
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
0.706 Å
The radius of the nth Bohr orbit for a hydrogen-like ion is given by r_n = a_0 × n^2 / Z, where a_0 = 0.529 Å is the first Bohr radius for hydrogen, n is the principal quantum number, and Z is the atomic number of the ion. This formula captures two competing effects: higher n increases the orbital radius while higher Z contracts it due to stronger nuclear attraction. For Li^{2+} with Z=3 and n=2: r_2 = 0.529 × (2)^2 / 3 = 0.529 × 4 / 3 = 2.116 / 3 = 0.705 Å ≈ 0.706 Å. Option 0.353 Å would correspond to r_1 of Li^{2+} (n=1): 0.529 × 1/3 = 0.176 Å — actually this value (0.353 Å) arises if n=1 and Z=1.5, which is not physical. Option 0.529 Å is the first Bohr radius for hydrogen (n=1, Z=1) and does not apply here. Option 1.058 Å would arise from using Z=1 (hydrogen) with n=2: 0.529 × 4/1 = 2.116 Å — or it could represent a factor-of-2 error. This problem is a direct application of Bohr's orbit radius formula extended to hydrogen-like ions, a core NCERT and JEE topic. Plausibility check: for Li^{2+} (Z=3) at n=2, the radius is 4/3 × 0.529 ≈ 0.706 Å, which is smaller than the hydrogen n=2 radius (4 × 0.529 = 2.116 Å) due to the higher nuclear charge, confirming the answer is physically reasonable.
This medium difficulty chemistry question is from the chapter atomic structure, covering the topic of bohr radius. It appeared in the 2025 exam.
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