Nuclear structure and nuclides
Key idea: A reviewed, static H2 Physics learning chain for all official Nuclear Physics outcomes, from Rutherford evidence to fusion and fission.
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The core idea
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Learn the idea
Big question: What does nuclide notation reveal about a nucleus?
A nucleus contains protons and neutrons: proton number Z identifies the element and nucleon number A counts both, so neutron number is A − Z. Isotopes share Z but differ in neutron number. Scattering evidence shows positive charge and most atomic mass concentrated in a tiny nucleus.
Turn each scattering observation into an inference
Most alpha particles crossed thin foil with little or no deflection, showing that most atomic volume is empty. Some deflected because positive alpha particles were repelled by positive charge.
A very small fraction turned through large angles. A strong force in a rare close encounter requires positive charge and most atomic mass to be concentrated in a tiny nucleus.
Check your understanding: Why does the rarity of backward scattering show that the nucleus is small?
Only particles passing very close to the concentrated repelling region turn back, and very few do so.
Read nuclide notation without mixing the two numbers
In ᴬ_ZX, Z is the proton number and identifies the element. A is the nucleon number, so neutron number is A − Z. A neutral atom has Z electrons, but electrons are not counted in A.
Isotopes share the same Z and therefore belong to one element, but have different neutron numbers and A values.
Check your understanding: How many neutrons are in ³⁷₁₇Cl?
37 − 17 = 20 neutrons.
Key ideas to keep
- Changing neutron number makes an isotope, not a different element.
- Nucleon number is not the same as relative atomic mass.
- Nuclear size is far smaller than atomic size.
See the reasoning
Worked example
Use scattering evidence and nuclide notation together
Question: A foil experiment records 99.3% of alpha particles within 1° of their original direction and 0.02% beyond 90°. Explain both observations. Then interpret ⁶⁰₂₇Co.
Step 1: Interpret the common result
Why: The dominant path describes most of the atom's volume.
Working: Almost all particles pass through, so most atomic volume is empty.
Step 2: Interpret the rare strong result
Why: A turn beyond 90° requires a strong repulsive interaction.
Working: Positive charge and most mass are concentrated in a very small nucleus encountered only rarely.
Step 3: Read the nuclide
Why: Z and A count different nuclear properties.
Working: Cobalt-60 has 27 protons and 60 − 27 = 33 neutrons.
Answer: The atom is mostly empty with a tiny positive massive nucleus; cobalt-60 contains 27 protons and 33 neutrons.
Check: The proton count matches cobalt's Z, while protons plus neutrons return A = 60.
Another worked model
Question
Connect the frequency of Rutherford deflections to nuclear size and charge.
Check the worked solution
Most alpha particles see little concentrated charge; rare close approaches experience strong repulsion and large deflection. Rarity bounds the positive region to a tiny fraction of atomic volume.
Use a hint if needed
Practise with support
Try this
For ³⁷₁₇Cl, state proton and neutron numbers.
Hint: Neutrons equal A − Z.
Check your answer
17 protons and 20 neutrons.
Now work without the hint
Practise independently
Your turn
Explain Rutherford evidence and distinguish nucleon number, proton number and isotopes.
Check your answer
Scattering supports a tiny positive nucleus and mostly empty atom. A counts protons plus neutrons; Z counts protons and identifies the element. Isotopes share Z but differ in neutron number and A.
Avoid these traps
Common mistakes
Common mistake
Isotopes have different proton numbers.
What is wrong with this reasoning?
Show better thinking
Isotopes share proton number and differ in neutron number.
Write for the examiner
Exam guidance
Check that nucleon and charge numbers balance whenever you write a nuclide symbol.
Exam-style practice [7 marks]
Explain how Rutherford scattering supports a nuclear atom, then distinguish the nuclides ³⁵₁₇Cl and ³⁷₁₇Cl.
Plan before you answer
- Match each scattering observation to an inference.
- Use the alpha particle's positive charge.
- Compare Z and A for the nuclides.
Mark your answer and compare the model
Marking points
Tick each point only if your answer states it clearly.
Model answer
Most alpha particles pass straight through, so most of the atom is empty. Positive alpha particles are repelled, and the rare large-angle turns require a strong force from positive charge concentrated in a very small nucleus that also contains most of the mass. Both chlorine nuclides have Z = 17, so each has 17 protons. They have 18 and 20 neutrons respectively and are isotopes of the same element.
Come back in three days
Check what stayed with you
Recall question 1
What does Z represent?
Check the answer
Proton number.
Recall question 2
What does A − Z give?
Check the answer
Neutron number.
Recall question 3
What observation supports a tiny nucleus?
Check the answer
The very small fraction of alpha particles scattered through large angles or backwards.
Syllabus and review details
This lesson covers the listed H2 Physics 9478 outcomes. Topic 20 excludes knowledge of positron emission in 20(g) and detailed knowledge of the antineutrino and particle zoo in 20(o). Nuclide equations conserve nucleon number, charge, mass-energy and momentum. Count data require background correction before population-law inference. Applications must relate half-life, penetration and ionisation to benefit and hazard. The binding-energy-per-nucleon curve, not a claim that mass disappears, explains fusion and fission energy release.
- GCE A-Level H2 PhysicsTopic 20(a) / Topic 20(b) / Topic 20(c) · 2027Checked against the syllabus · partial topic coverageOfficial 9478 syllabus
Course and syllabus information
- Course
- GCE A-Level H2 Physics
- Edition
- GCE A-Level H2 Physics 2027