Development of atomic modelsIB MYP Sciences: Revision notes

The atom is far too small to see, so scientists build models to explain what they observe. A model is a simplified picture or idea that explains evidence and can be used to make predictions. When new evidence cannot be explained, the model has to be changed. The atomic model has changed four main times, from Dalton to Bohr.

John Dalton suggested that all matter is made of tiny, solid spheres called atoms. He proposed that atoms cannot be divided, that all atoms of one element are identical, and that atoms of different elements are different.

This model explained why elements combine in fixed ratios, but it said nothing about what is inside an atom.

J. J. Thomson found that a beam in a tube was made of tiny negatively charged particles, which were much smaller than atoms. These were electrons. So atoms were not indivisible after all.

Because atoms are neutral, Thomson proposed the plum pudding model: a ball of positive charge with negative electrons embedded throughout it, like plums in a pudding.

In the gold foil experiment, alpha particles (positively charged) were fired at a very thin sheet of gold foil. The results were:

• most alpha particles passed straight through, so atoms are mostly empty space
• a few were deflected through large angles
• about 1 in 8000 bounced almost straight back

The plum pudding model could not explain this, because spread-out positive charge could only deflect the particles slightly. Rutherford concluded that all the positive charge and most of the mass is concentrated in a tiny, dense nucleus at the centre, with the electrons around it.

Niels Bohr suggested that electrons orbit the nucleus in fixed shells, or energy levels, at set distances. An electron cannot be between shells.

The evidence was that heated elements give out light of only certain colours. This fits electrons moving between fixed energy levels, and the nuclear model could not explain it. The Bohr model is the shell model that you use to write electron configurations such as 2,8,1.

A model is accepted because it explains the evidence available. When new evidence appears that the model cannot explain, scientists test a new model, and other scientists check the work before it is accepted.

• Dalton: solid sphere
• Thomson: electrons found, so plum pudding
• Rutherford: gold foil results, so a nucleus
• Bohr: coloured light, so shells

A changing model does not mean that science is unreliable. It shows that science improves as evidence grows. Older models can still be useful: the shell model is still used to explain how atoms react.