The history of the atomic model · GCSE Chemistry

Early ideas

Before electrons were discovered, atoms were thought to be tiny spheres that could not be divided (John Dalton's model, early 1800s).
J.J. Thomson discovered the electron in 1897. This led to the plum pudding model: a ball of positive charge with negative electrons embedded in it.

In the alpha particle scattering experiment (Geiger and Marsden, working with Rutherford, 1909), positively charged alpha particles were fired at thin gold foil.
Most went straight through, so most of the atom is empty space. A few were deflected, and a very few bounced back, so the mass is concentrated in a tiny, positively charged centre: the nucleus.
This nuclear model replaced the plum pudding model.

Niels Bohr adapted the nuclear model: electrons orbit the nucleus at specific distances (energy levels or shells). His calculations agreed with experimental observations.
Later experiments showed that the positive charge of a nucleus is made of smaller particles, each with the same positive charge: protons.
James Chadwick provided evidence for neutrons in 1932, about 20 years after the nucleus became an accepted idea.

New experimental evidence can lead to a scientific model being changed or replaced.
Models are accepted when they explain the observations and make predictions that are confirmed by experiments.

Plum pudding model: Thomson's model of the atom: a ball of positive charge with electrons embedded in it.
Alpha particle: A positively charged particle made of two protons and two neutrons.
Alpha scattering experiment: Firing alpha particles at gold foil, which showed atoms have a small, dense, positive nucleus.
Nuclear model: The model of the atom with a tiny positive nucleus surrounded by electrons.
Energy level: A fixed distance from the nucleus where electrons orbit; also called a shell.
John Dalton: Proposed that atoms were tiny spheres that could not be divided.
J.J. Thomson: Discovered the electron and proposed the plum pudding model.
Ernest Rutherford: Led the alpha scattering work that showed atoms have a nucleus.
Niels Bohr: Suggested that electrons orbit the nucleus in fixed energy levels.
James Chadwick: Found evidence for the neutron.