# Explain why electrons in an atom do not spiral inward and collapse into the nucleus, according to the Bohr model of the atom.

> IB DP Physics Standard Level (2023 syllabus) — E.1 Structure of the atom · Explain · 4 marks

> Classical physics predicted that an electron orbiting a nucleus would continuously radiate electromagnetic energy, lose speed, and spiral into the nucleus within a fraction of a second. The Bohr model was proposed to resolve this contradiction.

## Mark scheme (4 marks)

1. In the Bohr model, electrons are restricted to specific (discrete/quantised) orbits or energy levels.
2. An electron in an allowed orbit does not radiate energy, so it does not lose energy and does not spiral inward.
3. Energy is only emitted (or absorbed) when an electron transitions between two allowed energy levels.
4. There is a lowest allowed energy level (ground state) below which the electron cannot fall, preventing collapse into the nucleus.

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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