Development of the atomic model (Dalton onwards)

Predict, then check

Picture Thomson's plum pudding: a ball of positive charge with negative electrons embedded in it. Now Rutherford's group fires positive alpha particles at very thin gold foil, with a detector tracking where each one ends up.

Using the plum pudding model, what did Rutherford expect to happen to the alpha particles?

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Chemistry · Atomic structure

Whose idea was it?

Which scientist does each idea belong to?

8 of 8 still to sort.

What you need to know

• Atomic models have changed over time. Dalton (1803) said atoms are tiny, hard, solid spheres that cannot be divided.
• Thomson (1897) found atoms are not solid spheres. They contain smaller, negatively charged particles called electrons.
• His plum pudding model: electrons embedded in a ball of positive charge, stuck in position and not moving around.
• Have a goYour mate says Thomson's electrons whizz around the plum pudding like tiny planets. What do you tell them?

  In the plum pudding model the electrons are stuck in position and do not move around.

  The electrons are embedded in the ball of positive charge, so they stay fixed in place.

• In 1909, Rutherford and his research group fired positive alpha particles at very thin gold foil, with a detector tracking their paths.
• Rutherford expected every alpha particle to go straight through. Most did, a few rebounded straight back, and some were deflected.
• Have a goRutherford expected every alpha particle to go straight through. Which results did he not expect?

  A few rebounding straight back, and some being deflected at an angle.

  Only 'most went straight through' fits his expectation, and even that fell short of 'every one'.

• Those results proved the plum pudding model wrong. Most going straight through means the atom is mostly empty space.
• A few rebounding means a tiny, dense nucleus at the centre. Deflection means that nucleus is positively charged.
• The nuclear model: a small positive nucleus surrounded by a cloud of orbiting electrons, in an atom that is mostly empty space.
• Bohr (1913) adapted it: electrons orbit at fixed distances, in energy levels or shells. His calculations agreed with experiments.
• Rutherford showed the nucleus divides into positive protons. Chadwick (1932) found the neutron: similar mass, but no charge.
• Have a goCompare the neutron with the proton: what is the same, and what is different?

  Similar mass. The proton is positive; the neutron has no charge.

  Chadwick's particle matches the proton for mass, but it carries no charge, unlike the positive proton.

The big picture

The model of the atom was rebuilt several times: Dalton's solid spheres, Thomson's plum pudding, Rutherford's nucleus, Bohr's shells and Chadwick's neutron. Each change followed new evidence, and the gold foil experiment was the turning point.

⚠ Watch out

Thinking the atom was worked out once and never changed. Every model was replaced or refined when new evidence appeared. Also watch the mix-up between the two big models: plum pudding electrons are stuck in position, nuclear-model electrons move around the nucleus, and neither model had protons yet.

Memory hook

Ball, Pudding, Nucleus, Shells, Neutron: that is Dalton, Thomson, Rutherford, Bohr, Chadwick, in order. Each name is one step forced by new evidence.

Check yourself

Cover the page. Put the five scientists in order with their dates, say what each one changed, and explain which gold foil result showed there was a tiny, dense nucleus.