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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (4 marks)

In the Bohr model electrons can only occupy discrete, quantised orbits.

An electron in one of these allowed orbits does not radiate electromagnetic energy; it is stable.

Only when an electron jumps from one allowed orbit to another does it emit or absorb a photon.

Because there is a lowest allowed orbit (the ground state), the electron cannot lose energy beyond this point, so it cannot spiral into the nucleus.

Examiner tips

  • State that electrons occupy quantised orbits; mention stability of an allowed orbit; explain energy change only on transitions; note existence of a ground state to prevent collapse.

Common mistakes

  • Claiming electrons can orbit continuously; confusing classical radiation with Bohr stability; ignoring the ground state as a lower energy limit.

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.

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