Our understanding of the atom has evolved over time. Scientists used experiments and observations to build better models of atomic structure. Each new model replaced the one before it as evidence improved.

Dalton’s model (early 1800s)

John Dalton proposed that:

• All matter is made of tiny particles called atoms
• Atoms are solid spheres
• Different elements contain different types of atoms

Dalton believed atoms were indivisible and had no internal structure.

Thomson’s model (1897)

J.J. Thomson discovered the electron, a tiny negatively charged particle.

He proposed the plum pudding model:

• Atoms are spheres of positive charge
• Electrons are embedded inside like plums in a pudding

Rutherford’s model (1909)

Ernest Rutherford carried out the alpha particle scattering experiment.

The experiment:

• Fired positively charged alpha particles at a thin sheet of gold foil
• Most passed straight through, but some were deflected or bounced back

Conclusions:

• Atoms are mostly empty space
• They have a small, dense, positively charged nucleus
• Electrons orbit the nucleus

This disproved the plum pudding model and led to the nuclear model of the atom.

Bohr’s model (1913)

Niels Bohr improved Rutherford’s model by suggesting:

• Electrons orbit the nucleus at fixed energy levels (shells)
• These shells prevent electrons from spiralling into the nucleus

His model matched experimental results and is still used in GCSE-level chemistry.

Chadwick’s discovery (1932)

James Chadwick discovered the neutron, a neutral particle in the nucleus.

This explained the remaining mass in atoms that couldn’t be accounted for by protons and electrons alone.

Timeline of atomic models