11 min readPathway sheetSection A MCQ (1 mark) and objective (2 marks, two parts); Section B long answer (5 marks): the experiment, its findings and limits, often with a choice

Thomson's model, alpha scattering and Rutherford's nuclear model

Alpha particles fired at thin gold foil mostly passed straight through, but a few bounced back. So Rutherford put all the positive charge in a tiny central nucleus.

Thomson's modelPlum pudding model: a ball of positive charge with electrons placed in it, like seeds in a watermelon

Alpha particleA helium nucleus ⁴₂He: charge +2e, about 7300 times heavier than an electron

Sizes foundNucleus about 10⁻¹⁵ m across; atom about 10⁻¹⁰ m, so the atom is about 10⁵ times wider

Two limits of the nuclear modelIt cannot explain the stability of the atom or its line spectrum

The Geiger-Marsden experiment, step by step

On Rutherford's advice, his students Hans Geiger and Ernest Marsden fired alpha particles at gold foil.

1
A radioactive source S, inside a lead block, gives out alpha particles. Narrow slits make them a fine, straight beam (a collimated beam).
2
The beam falls on a very thin gold foil T. Gold can be beaten into extremely thin sheets.
3
A zinc sulphide (ZnS) screen catches the scattered particles. Each hit makes a tiny flash of light, seen through a microscope M.
4
The screen and microscope rotate on a circular scale round the foil, so flashes are counted at every angle.
5★
The whole set-up is kept in an evacuated chamber (air removed), so air molecules do not scatter the alpha particles.

★ key step

What they saw and what it means

Observation	Conclusion
Most alpha particles went straight through or bent very little	Most of the atom is empty space
A few were turned through large angles	There is a strong repulsive push from a small, positive, heavy core
About 1 in 8000 bounced back, through more than 90°; a rare few came almost straight back (180°)	Only a head-on approach to this core turns a particle back, and that is rare

From Thomson's model to Rutherford's nuclear model

In 1897, J.J. Thomson found the electron. An atom is neutral, so it must also hold an equal positive charge. Thomson pictured a ball of positive charge with electrons placed in it: the plum pudding model. It predicts only tiny deflections of alpha particles. To explain the large-angle scattering, Rutherford (1911) put all the positive charge and almost all the mass in a tiny nucleus, about 10⁻¹⁵ m across. Electrons revolve round it, and the atom is neutral overall. The closer an alpha particle comes to the nucleus, the stronger the Coulomb repulsion and the larger its deflection.

Two limits of Rutherford's model

It cannot explain stabilityAn electron in a circular orbit is accelerating, because its direction keeps changing. Classical electromagnetic theory says an accelerated charge radiates energy. So the electron should lose energy and spiral into the nucleus in a tiny fraction of a second. Real atoms are stable.

It cannot explain line spectraWhile spiralling in, the electron's frequency of revolution would change smoothly. It would give out light of all frequencies: a continuous spectrum. Real hydrogen gives out only certain sharp wavelengths: a line spectrum.

Even so, the nuclear model was the first big step in the right direction. Bohr built his model on it.

Worked example: how empty is an atom?

Problem

The radius of a nucleus is about 10⁻¹⁵ m and the radius of an atom is about 10⁻¹⁰ m. (a) How many times wider is the atom? (b) If the nucleus were a marble 1 cm across, how wide would the atom be? (c) What fraction of the atom's volume does the nucleus fill?

Working

(a) Ratio of sizes = 10⁻¹⁰ m ÷ 10⁻¹⁵ m = 10⁵.
(b) Atom = 1 cm × 10⁵ = 10⁵ cm = 1000 m = 1 km across.
(c) Volume goes as the cube of size: (10⁵)³ = 10¹⁵.
So the nucleus fills only about 1 part in 10¹⁵ of the atom's volume.

Answer

(a) About 10⁵ times; (b) about 1 km; (c) about 10⁻¹⁵ of the volume, so the atom is almost all empty space

How to draw the apparatus and the paths

Draw a small box on the left labelled source S (alpha particles) inside a lead block, with a narrow slit.
Draw a straight arrow for the beam reaching a thin vertical line in the middle labelled gold foil T.
Draw a circle round the foil labelled circular scale. On it, draw a small ZnS screen with a microscope M behind it.
Draw a dashed box round everything labelled evacuated chamber.
For the paths: draw a dot labelled nucleus (+). Lines passing far away go almost straight, lines passing close bend sharply, and a line aimed at the nucleus turns back.

In real life

If the nucleus of an atom were a marble on the centre spot of a football ground, the electrons would be about half a kilometre away. Your table, your phone and your body are mostly empty space, held firm by electric forces.

Exam angle

5 marks: “Describe Rutherford's alpha particle scattering experiment. State its observations, conclusions and two limitations of the nuclear model.” Set-up: alpha source in lead block, slits, thin gold foil, rotating ZnS screen with microscope, vacuum. Most pass straight (atom mostly empty); a few deflected through large angles and about 1 in 8000 bounce back (more than 90°) (tiny, heavy, positive nucleus about 10⁻¹⁵ m). Limits: revolving electron should radiate and spiral in; predicts a continuous spectrum, but atoms give line spectra.

2 marks: “Most alpha particles went through the gold foil without turning. What does this tell us about the atom?” Most of the atom is empty space. Most alpha particles pass far from any nucleus, so the repulsion on them is too weak to deflect them.

1 marks: “Which result of the scattering experiment could be explained only by a nucleus?” A few alpha particles were scattered through large angles, some almost back along their path.

Remember it

Most through, few bent, rare back: an empty atom with a tiny, heavy, positive core.