Ideas about atoms have changed over time. Scientists develop new atomic models as they gather new experimental evidence.
John Dalton published his ideas about atoms in 1804. He thought that all matter was made of tiny particles called atoms, which he imagined as tiny spheres that could not be divided. Dalton used his atomic model to explain the properties of gases and the formulae of compounds.
In 1897, J J Thomson carried out experiments and discovered the electron. The mass of an electron was much smaller than the mass of an atom, so electrons must be subatomic particles. This led him to suggest the plum pudding model of the atom. In this model, the atom is a ball of positive charge with negative electrons embedded in it.
In 1911 Ernest Rutherford used results from an experiment to test the plum pudding model. In the experiment, positively charged alpha particles were fired at thin gold foil. Most alpha particles went straight through the foil. But a few were scattered in different directions.
This evidence led Rutherford to suggest a new model for the atom, called the nuclear model. In the nuclear model:
Further experiments led to the idea that the nucleus contained small particles, called protons. Each proton has a small amount of positive charge.
In 1932 James Chadwick found evidence for the existence of particles in the nucleus with mass but no charge. These particles are called neutrons. This led to another development of the atomic model, which is still used today.