The development of the model of the atom (common content with physics): Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations.

The idea

Before Bohr, the nuclear model had a serious flaw: classical physics predicted that an orbiting electron should radiate energy continuously, spiral inwards and make atoms collapse, yet atoms are stable and emit sharp line spectra. Bohr adapted the model by proposing that electrons orbit the nucleus only at specific distances, called energy levels or shells, and that each allowed orbit corresponds to a particular energy. Electrons can move between levels by absorbing or emitting a fixed amount of energy, often as electromagnetic radiation of a definite frequency. Crucially, Bohr's theoretical calculations matched experimental observations, such as the lines seen in hydrogen's emission spectrum, so his adapted model was accepted and replaced earlier ideas.

What earns the marks

• Bohr adapted the existing nuclear model rather than replacing the nucleus; the nucleus stays at the centre and electrons occupy the space around it.
• Electrons are restricted to specific distances from the nucleus, described as fixed orbits, shells or energy levels, not any distance.
• Each allowed orbit has a fixed energy, so an electron can only have certain energy values.
• An electron changes orbit by absorbing energy to move further out or emitting energy, often as electromagnetic radiation, to move closer in.
• Bohr's theoretical calculations agreed with experimental observations, for example the sharp lines in the hydrogen emission spectrum, which supported the model.

Common mistakes and how to put them right

• Saying electrons orbit at any distance.

  Put it right by stressing that only specific, allowed orbits or energy levels exist.

• Confusing Bohr's change with the discovery of the nucleus.

  Put it right by stating that Rutherford's group established the nuclear model and Bohr adapted it by fixing electron orbits.

• Claiming Bohr proved electrons are particles called protons.

  Put it right by separating Bohr's electron-orbit idea from later work on subatomic particles.

Exam tips

• Link the word 'specific' to fixed energy levels, and use the phrase 'agreed with experimental observations' when explaining why the model was accepted.
• If asked to compare models, describe one clear difference, such as electrons in fixed orbits rather than anywhere, and one piece of supporting evidence.
• Use a labelled diagram of shells around a nucleus to make the idea of specific distances clear in a written answer.