What is meant by nucleon number?

The total number of protons and neutrons in the nucleus (symbol A).

Nuclei with the same proton number but different numbers of neutrons (different nucleon numbers).

Describe the plum pudding model.

A sphere of positive charge with electrons embedded in it; the positive charge is spread through the whole atom.

Why was the alpha scattering apparatus evacuated?

Air would absorb or scatter the alpha particles, which have a short range in air.

What did the observation that most alpha particles passed straight through show?

That most of the atom is empty space.

What did large-angle deflections show?

There is a small region of concentrated positive charge (the nucleus) that repels the alpha particles.

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

The nucleus is very small and contains most of the mass of the atom.

Approximately what is the radius of a nucleus compared with an atom?

About 10⁻¹⁵ m for the nucleus compared with about 10⁻¹⁰ m for the atom.

Why could the plum pudding model not explain alpha scattering?

With charge spread through the atom the forces on alpha particles would be small, so it predicts only small deflections.

What did Chadwick discover in 1932 and why was it important?

The neutron; it explained why the mass of a nucleus is greater than that of its protons alone.

What did Bohr add to Rutherford's model?

Electrons in fixed orbits (discrete energy levels), which explained atomic line spectra.

Atomic structure and alpha scatteringEdexcel International A Level Physics: Flashcards

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What is meant by proton number?

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What is meant by proton number?: The number of protons in the nucleus (symbol Z); it identifies the element.
What is meant by nucleon number?: The total number of protons and neutrons in the nucleus (symbol A).
How do you find the number of neutrons from A and Z?: N = A − Z.
What are isotopes?: Nuclei with the same proton number but different numbers of neutrons (different nucleon numbers).
Describe the plum pudding model.: A sphere of positive charge with electrons embedded in it; the positive charge is spread through the whole atom.
Why was the alpha scattering apparatus evacuated?: Air would absorb or scatter the alpha particles, which have a short range in air.
What did the observation that most alpha particles passed straight through show?: That most of the atom is empty space.
What did large-angle deflections show?: There is a small region of concentrated positive charge (the nucleus) that repels the alpha particles.
What did the few alpha particles scattered back through more than 90° show?: The nucleus is very small and contains most of the mass of the atom.
Approximately what is the radius of a nucleus compared with an atom?: About 10⁻¹⁵ m for the nucleus compared with about 10⁻¹⁰ m for the atom.
Why could the plum pudding model not explain alpha scattering?: With charge spread through the atom the forces on alpha particles would be small, so it predicts only small deflections.
What did Chadwick discover in 1932 and why was it important?: The neutron; it explained why the mass of a nucleus is greater than that of its protons alone.
What did Bohr add to Rutherford's model?: Electrons in fixed orbits (discrete energy levels), which explained atomic line spectra.

Exam questions on Atomic structure and alpha scattering

Copper has two stable forms of atom, copper-63 and copper-65. The nuclide notation for copper-63 is $^{63}_{29}\mathrm{Cu}$ .
Explain why atoms of copper-63 and copper-65 have the same chemical properties but different masses.2 marks
In 1909 Geiger and Marsden directed a narrow beam of alpha particles at a very thin gold foil inside an evacuated chamber. A detector was moved around the foil to count the alpha particles scattered through different angles.
A very small number of alpha particles were scattered through angles greater than 90°. Explain what this shows about the structure of the atom.2 marks
In a repeat of the alpha particle scattering experiment, 2.4 × 10⁵ alpha particles are incident on a thin gold foil. About 1 in 8000 of the alpha particles is deflected through an angle greater than 90°. Before this experiment the plum pudding model proposed by J. J. Thomson was the accepted model of the atom.
Calculate the number of alpha particles deflected through more than 90°, and state what the result shows about the size of the nucleus compared with the atom.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).