GCSE · Chemistry · AQA · Spec 8462

Development of the model of the atom

How the atomic model changed

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

The crucial model change

Plum pudding modelvsNuclear model

Compare the model before and after the scattering evidence.

Focus

Where the mass is pictured

Plum pudding model

The model does not concentrate the atom's mass in a central nucleus.

Nuclear model

The atom's mass is concentrated at the centre in the nucleus.

The insight

Scattering evidence required a model with the atom's mass concentrated at the centre.

Positive charge

Plum pudding model

Positive charge forms a ball with electrons embedded in it.

Nuclear model

The centre of the atom contains a charged nucleus.

Electrons

Plum pudding model

Negative electrons are embedded in the positive ball.

Nuclear model

The nuclear model replaced the plum pudding picture after the scattering evidence.

Why the model changed

Plum pudding model

This model was the accepted picture after the electron was discovered.

Nuclear model

This model replaced it because the scattering results showed concentrated central mass and a charged nucleus.

How evidence changes a model

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

  1. Start with a modelThe plum pudding model pictured a ball of positive charge with negative electrons embedded in it.
  2. Collect new evidenceThe alpha-particle scattering experiment produced evidence about where the atom's mass and charge are concentrated.
  3. Find the mismatchThe evidence showed that mass is concentrated at the centre in a charged nucleus, which did not fit the plum pudding picture.
  4. Replace the modelThe 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?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Follow how new evidence repeatedly changed scientists' picture of the atom.

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