Atomic models
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 elementA substance made of one type of atom only.. Later Greek thinkers suggested that matterSub-atomic particles and anything made from them, such as atoms and molecules, are matter. Energy and forces are not matter. could be made up of invisible particleA general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules.. They called these particles atomThe smallest part of an element that can exist. but they had no experimental evidence for their model.
Learn more on the history of the atom in this podcast.
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The first atomic 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鈥檚 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
- the atoms of different elements join to make compoundA substance formed by the chemical union of two or more elements.
- during chemical reactions atoms rearrange to make different substances
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.
A timeline to show how the atomic model has changed
| 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. |
| Year | 1897 |
|---|---|
| Scientist(s) | Thomson |
| New evidence | The discovery of electrons. |
| Changes to the atomic model | 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. |
| Year | 1909-1911 |
|---|---|
| Scientist(s) | Rutherford (and Geiger and Marsden) |
| New evidence | Some positively charged particles fired at gold foil bounced back when they were expected to pass straight through. |
| Changes to the atomic model | Atoms have a central positive nucleus. Most of the mass of an atom is found in the nucleus. |
| Year | 1913 |
|---|---|
| Scientist(s) | Bohr |
| New evidence | In-depth work on Rutherford's model showed it had limitations. The electrons should just spiral in towards the positive nucleus. |
| Changes to the atomic model | Electrons move in fixed orbits, called electron shells, around the nucleus. |
Scientists then discovered that the nucleusThe central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei. is made up of two types of subatomic particleObjects found in atoms, including protons, neutrons and electrons. called protonSubatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1. and neutronUncharged subatomic particle, with a mass of 1 relative to a proton. The relative charge of a neutron is 0..
As a result of these discoveries, we now know that:
- atoms can be broken down into three smaller particles (protons, neutrons and electronSubatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.)
- 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