12 min readConcept sheetSection A MCQ (1 mark) and objective (2 marks, two parts); Section B long answer (5 marks): the experiment with a diagram, the model and its drawbacks, often with a choice

Rutherford's alpha scattering experiment, the nuclear model and its drawbacks

Alpha particles fired at thin gold foil mostly went straight through, but a few bounced back. So an atom is mostly empty space around a tiny, dense, positive nucleus.

Alpha particleA helium nucleus, He²⁺: 2 protons and 2 neutrons, charge +2

The experimentGeiger and Marsden (1910) fired alpha particles at gold foil about 0.00004 cm thick; Rutherford explained the results in 1911

NucleusTiny, dense, positive centre holding nearly all the mass; its radius is at least 10 000 times smaller than the atom's

DrawbacksCould not explain the stability of the atom, the arrangement of electrons, or why atoms are heavier than their protons alone

The alpha scattering experiment

1
A radioactive source gives a narrow beam of fast alpha particles (helium nuclei, charge +2).
2
The beam strikes a very thin gold foil, about 0.00004 cm thick.
3
A fluorescent screen around the foil flashes wherever an alpha particle hits it, so the paths can be counted.
4★
Geiger and Marsden counted the flashes at every angle (1910). Rutherford explained the results in 1911.

★ key step

What they saw and what it means

Observation	Conclusion
Most alpha particles went straight through	Most of the atom is empty space
Some turned through small angles	They passed near a positive region that pushed them away
A few turned through large angles	That positive region is dense and strongly charged
About 1 in 12 000 bounced back	The positive charge and the mass sit in a tiny centre: the nucleus

Rutherford's nuclear model

At the centre of the atom is a nucleus: tiny, dense and positive.
The nucleus holds all the positive charge and nearly all the mass of the atom.
The electrons move around the nucleus, in the large empty space outside it.
The nucleus is tiny: its radius is at least 10 000 times smaller than the atom's. If an atom were a cricket stadium, the nucleus would be a pea or smaller at the centre.
The atom is neutral: the positive charge of the nucleus equals the total negative charge of the electrons.

Drawbacks of Rutherford's model

Stability: a charged electron moving in a circle should keep giving out energy as radiation. It would slow down, spiral into the nucleus, and the atom would collapse. Real atoms are stable, so the model is incomplete.
Arrangement of electrons: the model does not say how the electrons are arranged around the nucleus.
Mass: a helium atom has 2 protons, yet it is 4 times as heavy as a hydrogen atom (1 proton). With only protons in the nucleus, the model cannot explain this. The neutron later solved it.

Activity: Rutherford's scattering experiment (online lab)

Open an online simulation of Rutherford scattering on a phone or computer.

1
Choose the Thomson (plum pudding) atom first, and fire alpha particles at it.
Record: almost all go straight through, with tiny deflections.
2
Switch to the Rutherford (nuclear) atom and fire again.
3
Watch the particles that pass close to the nucleus.
Record: they swing away sharply, and one aimed straight at the nucleus turns back.
4
If the simulation allows, give the nucleus more protons and fire again.
Record: the deflections become larger.
5★
Compare the two atoms and write which model fits the gold foil results.

★ key step

How to draw the experiment and the paths

Draw a small box on the left labelled radioactive source (alpha particles) inside a lead block with a narrow slit.
Draw a straight arrow (the beam) to a thin vertical line labelled thin gold foil.
Draw a circle round the foil labelled fluorescent screen.
For the paths, draw a dot labelled nucleus (+). Lines far from it go straight on, lines near it bend, and one line aimed at it turns back.
Label the paths: most pass straight, some deflected, a few rebound.

In real life

Rutherford's discovery of the nucleus opened the way to nuclear science. India's nuclear power stations, such as those at Tarapur and Kudankulam, produce electricity from energy stored in nuclei.

Exam angle

1 marks: “According to your NIOS lesson, about how many alpha particles bounced back in the gold foil experiment?” About 1 in every 12 000.

5 marks: “With a labelled diagram, explain how the gold foil experiment led Rutherford to the idea of a nucleus. State two drawbacks of his model.” Set-up: a narrow beam of alpha particles from a radioactive source falls on thin gold foil, inside a fluorescent screen (diagram: source, beam, foil, screen and paths). Most particles go straight through, so the atom is mostly empty. Some bend, and about 1 in 12 000 bounce back, so the positive charge and the mass are packed into a tiny nucleus. Model: a positive nucleus at the centre, with electrons moving around it. Drawbacks: it cannot explain why the atom is stable, since a circling electron should spiral into the nucleus; and it does not give the arrangement of electrons.

2 marks: “Why did Rutherford conclude that most of the atom is empty space?” Most alpha particles passed straight through the gold foil without any deflection. Nothing was in their way, so most of the space in an atom must be empty.

Remember it

Most pass (empty space), some swerve (positive charge), a few rebound (tiny, dense nucleus).