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Chemistry Paper 1 / Atomic structure and the periodic table
Lesson 1.5 · AQA Combined Science

Development of
the atomic model

A scientific model is an explanation of how something works. Scientists change models when new evidence does not fit.
01 · Dalton

A solid sphere

Dalton described atoms as tiny, solid spheres that could not be divided.

Solid-sphere model.
02 · Thomson

Plum pudding

Thomson discovered the electron: a tiny negative particle. Atoms could be divided into smaller particles.

His model had negative electrons embedded in a sphere of positive charge. There was no nucleus.

The + represents positive charge spread throughout the sphere. The − signs are electrons.
03 · Rutherford

A tiny nucleus

The alpha-scattering results led Rutherford to propose a small, dense, positive nucleus containing most of the atom’s mass.

Electrons are outside the nucleus. Most of the atom is empty space.

Positive charge is concentrated in the nucleus. The dashed edge shows the atom’s extent, not a shell.
The evidence · Gold foil experiment

Watch the alpha particles

Geiger and Marsden fired positive alpha particles at thin gold foil, under Rutherford’s direction.

Press Play to watch. No sound is needed. The diagram is simplified and not to scale.

Most passed straight through.

The atom is mostly empty space.

Some changed direction.

A small positive nucleus repels the positive alpha particles.

Very few scattered backwards.

Most of the mass is concentrated in a tiny, dense nucleus.

The plum-pudding model could not explain the large deflections. The evidence led to a new model.
04 · Bohr

Electron shells

Bohr suggested that electrons occupy shells at specific distances from the nucleus.

His calculations agreed with observations.

Shells represent electron energy levels.
05 · Chadwick

The neutron

Later work identified positive particles called protons inside the nucleus.

Chadwick then provided evidence for neutrons in the nucleus. Neutrons have no electrical charge.

Enlarged nucleus: positive protons and neutral neutrons.

All model diagrams are simplified, not to scale, and do not represent a particular element.

Remember the sequence

Dalton: solid sphere.

Thomson: electrons in positive charge.

Rutherford: tiny positive nucleus.

Bohr: electron shells.

Chadwick: neutrons.