Ancient Greek thinkers believed that all matter was made of a combination of earth, air, fire and water. These substances were called 'elements' but they were not the same as modern elements. Later Greek thinkers suggested that matter could be made up of invisible particles. They called these particles atoms but they had no experimental evidence for their model.
The scientist John Dalton carried out a series of experiments. He concluded that all matter was made of tiny particles called atoms. He suggested that an atom was a tiny solid ball. He published his ideas in 1803.
Dalton’s model included these ideas:
This atomic model has changed over time. Scientists used the model to make predictions. Sometimes the results of their experiments were a surprise and they did not fit with the existing model. Scientists changed the model so that it could explain the new evidence.
|Year||Scientist(s)||New evidence||Changes to the atomic model|
|1897||Thomson||The discovery of electrons.||Atoms can be broken down into smaller parts. An atom is made of tiny negatively charged electrons dotted about a positively charged sphere like a plum pudding.|
|1909-1911||Rutherford (and Geiger and Marsden)||Some positively charged particles fired at gold foil bounced back when they were expected to pass straight through.||Atoms have a central positive nucleus. Most of the mass of an atom is found in the nucleus.|
|1913||Bohr||In-depth work on Rutherford's model showed it had limitations. The electrons should just spiral in towards the positive nucleus.||Electrons move in fixed orbits, called electron shells, around the nucleus.|
As a result of these discoveries, we now know that: