What is the proton number?

The number of protons in a nucleus (symbol Z); it defines the element.

How do you find the number of neutrons in a nucleus?

N = A − Z (nucleon number minus proton number).

Protons and neutrons, the particles in the nucleus.

Describe the plum pudding model.

A sphere of positive charge with electrons embedded in it.

Describe the alpha-scattering experiment.

A narrow beam of alpha particles was fired at a thin gold foil in a vacuum, and the scattering at different angles was counted.

Why was the alpha-scattering apparatus evacuated?

Alpha particles would be absorbed or scattered by air molecules.

Why was the gold foil very thin?

So that each large deflection was due to a single interaction with one nucleus.

What did most alpha particles do, and what does that show?

They passed straight through, showing that most of the atom is empty space.

What did the few alpha particles deflected through more than 90° show?

The atom has a small, dense, positively charged nucleus.

How did the experiment show the nucleus is positive?

Positive alpha particles were repelled, since like charges repel.

How did the experiment show the nucleus contains most of the mass?

Alpha particles rebounded almost straight back, so they hit something much more massive.

Why could the plum pudding model not explain alpha scattering?

Its spread-out positive charge could only give weak forces and small deflections, not large angles.

Why do scientists change models of the atom?

New experimental evidence cannot be explained by the existing model.

The nuclear atom and alpha scatteringEdexcel A-Level Physics: Flashcards

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What is the nucleon number?

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What is the nucleon number?: The total number of protons and neutrons in a nucleus (symbol A).
What is the proton number?: The number of protons in a nucleus (symbol Z); it defines the element.
How do you find the number of neutrons in a nucleus?: N = A − Z (nucleon number minus proton number).
What are nucleons?: Protons and neutrons, the particles in the nucleus.
Describe the plum pudding model.: A sphere of positive charge with electrons embedded in it.
Describe the alpha-scattering experiment.: A narrow beam of alpha particles was fired at a thin gold foil in a vacuum, and the scattering at different angles was counted.
Why was the alpha-scattering apparatus evacuated?: Alpha particles would be absorbed or scattered by air molecules.
Why was the gold foil very thin?: So that each large deflection was due to a single interaction with one nucleus.
What did most alpha particles do, and what does that show?: They passed straight through, showing that most of the atom is empty space.
What did the few alpha particles deflected through more than 90° show?: The atom has a small, dense, positively charged nucleus.
How did the experiment show the nucleus is positive?: Positive alpha particles were repelled, since like charges repel.
How did the experiment show the nucleus contains most of the mass?: Alpha particles rebounded almost straight back, so they hit something much more massive.
Why could the plum pudding model not explain alpha scattering?: Its spread-out positive charge could only give weak forces and small deflections, not large angles.
Why do scientists change models of the atom?: New experimental evidence cannot be explained by the existing model.

Exam questions on The nuclear atom and alpha scattering

A neutral atom of copper-63 is written ⁶³₂₉Cu.
Another isotope of copper has a nucleon number of 65. State the number of protons and the number of neutrons in its nucleus, and explain why it is still copper.2 marks
In the Geiger–Marsden experiment a narrow beam of alpha particles in an evacuated chamber was directed at a very thin gold foil. A detector that could be moved round the foil counted the alpha particles scattered at different angles. Most alpha particles passed through the foil with little or no deflection, but a very small number were deflected through large angles.
Explain why the gold foil had to be very thin.2 marks
In one run of an alpha-scattering experiment with a thin gold foil, a total of 4.0 × 10⁵ alpha particles were counted. About 1 in 8000 of them was scattered through an angle of more than 90°.
Calculate the number of alpha particles scattered through more than 90°, and state what such a small fraction shows about the nucleus.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).