Development of the atomic model

Our model of the atom has changed as new evidence was found.

In the early 1800s, John Dalton described atoms as tiny solid spheres that could not be divided.

In 1897, J.J. Thomson discovered the electron. This showed atoms contain even smaller particles, so they cannot be solid spheres. He suggested the plum pudding model: a ball of positive charge with negative electrons spread through it.

In 1909, Geiger and Marsden, working with Ernest Rutherford, fired positive alpha particles at very thin gold foil.

• Most alpha particles went straight through, so most of the atom is empty space.
• A few were deflected, and a very small number bounced back. Something small, dense and positively charged must be repelling them.

The plum pudding model could not explain this. Rutherford replaced it with the nuclear model: almost all the mass is in a tiny, positive nucleus at the centre, with the electrons around it.

Around 1913, Niels Bohr suggested that electrons orbit the nucleus at fixed distances, in shells or energy levels. His calculations agreed with experimental results.

Later experiments showed that the positive charge of the nucleus is shared between smaller particles, each with the same charge. These are protons.

In 1932, James Chadwick provided evidence for neutrons: particles in the nucleus with mass but no charge. This explained why the mass of atoms was larger than the protons alone could account for.