FRQ — Structure of Atom

Short answer·Standard multi-step (3/5)·No calculator

Difficulty guide

The label describes the problem, not your ability.

1/5 - Foundational

Warm-up setup; checks one basic idea.

2/5 - Routine exam skill

Normal syllabus skill; usually one main rule or interpretation.

3/5 - Standard multi-step

Exam-style problem with multiple steps or a common trap.

4/5 - Hard exam skill

High-end exam problem; combines methods carefully.

5/5 - Challenge

Stretch problem beyond normal exam pressure.

Question

According to Bohr's model,

r_n=n^2a_0/Z

for a hydrogen-like species. Compare the radius of the first orbit of

\mathrm{He^+}

with the first orbit of hydrogen.

Total: 3 marks

(a) Find $r_1$ for hydrogen in terms of $a_0$ . (1 mark)

(b) Find $r_1$ for $\mathrm{He^+}$ in terms of $a_0$ . (1 mark)

Your response (optional — you can also work on paper)

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