Development of the model of the atom

What you need to know

• New experimental evidence can cause a scientific model to be changed or replaced.
• Atoms were first pictured as tiny indivisible spheres, then as a positive ball containing embedded negative electrons.
• Alpha-particle scattering showed that atomic mass is concentrated in a charged central nucleus, replacing the plum pudding model with the nuclear model.
• Bohr proposed electrons at specific distances from the nucleus; later experiments identified protons and Chadwick provided evidence for neutrons in the nucleus.

The big picture

Scientific models can be changed or replaced when new experimental evidence appears. The atomic model developed from tiny indivisible spheres to the plum pudding model, then to a nuclear model after alpha-particle scattering evidence. Niels Bohr adapted the nuclear model by proposing electrons at specific distances from the nucleus, supported by calculations that agreed with observations. Later evidence established protons and then neutrons within the nucleus.

How the atomic model changed

Follow the sequence and notice what new evidence forced each change.

How evidence changes a model

Use the scattering experiment as the clearest example of scientific model change.

1. 1

   Start with a model

   The plum pudding model pictured a ball of positive charge with negative electrons embedded in it.

2. 2

   Collect new evidence

   The alpha-particle scattering experiment produced evidence about where the atom's mass and charge are concentrated.

3. 3

   Find the mismatch

   The evidence showed that mass is concentrated at the centre in a charged nucleus, which did not fit the plum pudding picture.

4. 4

   Replace the model

   The plum pudding model was replaced by the nuclear model because the new model matched the experimental evidence.

Models and evidence

Keep the names tied to the change they describe.

Predict, then check

Use the scattering evidence rather than recalling the model name alone.

If an experiment shows that most of an atom's mass is concentrated at its centre in a charged region, which conclusion best follows?

Memory hook

Sphere → Pudding → Nucleus → Orbits → Protons → Neutrons: follow the evidence as the model gains detail.

Check yourself

Why was the alpha-particle scattering experiment a turning point in the development of the atomic model?