Answer: 2.12 Å.
- A 0.26 Å
- B 1.06 Å
- C 2.12 Å
- D 0.53 Å
Correct answer: C. 2.12 Å
Explanation: r<sub>n</sub> = n²·a₀ = 4·0.53 = 2.12 Å.

The Geiger–Marsden alpha-scattering result: a few alpha particles bounce back at large angles, which rules out Thomson’s diffuse model and demands a tiny, massive, positively charged nucleus. Image: Kurzon, CC BY 3.0, via Wikimedia Commons.
Concept context
Thomson's plum-pudding model, Rutherford's nuclear model from alpha-scattering, Bohr's postulates and hydrogen spectrum, spectral series (Lyman to Pfund), Rydberg formula, and limitations of each model.