6.17 - Evidence for atomic models

A model makes a prediction

A scientific model is a representation used to explain observations and make predictions. It is not a claim that an atom literally looks like a classroom picture. A model remains useful only while its predictions agree with the available evidence.

After the electron was discovered, the plum pudding model described an atom as a sphere of spread-out positive charge with negative electrons embedded within it. The positive charge was not concentrated in a nucleus. This model explained why an atom could be neutral, but it made a testable prediction: an energetic, positively charged alpha particle passing through a thin metal foil should experience only small deflections from the spread-out charge.

Scattering evidence and the nucleus

Rutherford's team directed alpha particles at very thin gold foil and observed where the particles travelled:

• Most passed straight through. This suggested that most of an atom is empty space.
• Some were deflected through an angle. This suggested that positive charge is concentrated, because a positive alpha particle is repelled when it passes near the concentrated positive region.
• A very small number were deflected through large angles or came back. This required a tiny, dense region containing most of the atom's mass and its positive charge.

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