Answer: 4.10 10 -15 s.
- A 4.56 × 10<sup>-16</sup> s
- B 1.37 × 10<sup>-15</sup> s
- C 9.12 × 10<sup>-15</sup> s
- D 4.10 × 10<sup>-15</sup> s
Correct answer: D. 4.10 × 10<sup>-15</sup> s
Explanation: The Bohr orbital period varies as T proportional to n<sup>3</sup>. Therefore T<sub>3</sub> = 3<sup>3</sup>T<sub>1</sub> = 27 × 1.52 × 10<sup>-16</sup> = 4.10 × 10<sup>-15</sup> s.

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.