Developing models of atoms
John Dalton published his ideas about atoms in 1803. He thought that all matter was made of tiny particles called atoms, which he imagined as tiny solid balls. Dalton's model included these ideas:
- atoms cannot be broken down into anything simpler
- the atoms of a given element are identical to each other
- the atoms of different elements are different from one another
- during chemical reactions atoms rearrange to make different substances
The Dalton model has changed over time because of the discovery of subatomic particles.
A timeline of discovery
|1909 - 1911||Atoms have a nucleus with electrons around it|
|1913||Electrons occupy shells (energy levels)|
As a result of these discoveries, we now know that:
- atoms can be broken down into three smaller particles: protons, neutrons and electrons
- atoms of an element have identical numbers of protons and electrons, but can have different numbers of neutrons
- atoms of different elements have different numbers of protons and electrons
Remember that in Dalton's time, the technology did not exist to discover the detailed structure of atoms.