Nuclide Notation
Key idea: Learn nuclide notation ^A_ZX, find protons/neutrons/electrons, and balance simple nuclear equations by conserving A and Z in exam questions (O Level).
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The core idea
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Learning objectives
- Describe atomic composition
- Use proton number, nucleon number and isotope
- Use and interpret nuclide notation
- Explain random and spontaneous nuclear decay
- Describe alpha, beta-minus and gamma radiation
- Compare ionising effect and penetrating power
- Use nuclide equations for radioactive decay
- Explain background radiation
- Use half-life in tables and decay curves
- State radioactivity applications and hazards
- Evaluate uses and hazards using half-life and radiation properties
- Relate fission and fusion to nuclear-energy release
1. Definition
A. Nuclide notation
Nuclide notation is written as:
^A_ZX
where:
- X is the element symbol
- Z is the proton number
- A is the nucleon number (protons + neutrons)
B. Nuclide
A nuclide is an atom type with a specific nucleus (a specific number of protons and neutrons).
Read ^A_ZX in this order: use Z to identify the element, then calculate the neutron number from A-Z. The two numbers describe the nucleus, not the electron arrangement.
You should be able to use nuclide notation and balance simple nuclear equations by conserving A and Z.
2. Key Ideas
- Neutrons: N = A - Z.
- For a neutral atom:
- protons = Z
- electrons = Z
- neutrons = A - Z
- In nuclear equations, total A and total Z are conserved.
- If the species is an ion, its electron number differs from Z; the nuclide symbol still describes the same nucleus.
Common emissions:
- alpha: ⁴₂He
- beta (β−): ⁰₋₁e
- gamma: ⁰₀γ (no change to A or Z)
3. Detailed Explanations
A. What A and Z tell you
- Z tells you the element (same Z = same element).
- A tells you how many particles are in the nucleus (nucleons).
B. Balancing nuclear equations (method)
To find the unknown nuclide:
- Write the total nucleon number A on each side.
- Write the total proton-number value Z on each side, including any emitted particle.
- Subtract to find the unknown A and Z.
- Use the resulting Z to identify the element symbol if a periodic table is provided.
4. Common Mistakes
- Using A as the number of neutrons (neutrons are A-Z).
- Balancing A but forgetting to balance Z.
- Writing β− as ⁰₊₁e (β− is an electron: ⁰₋₁e).
- Changing A during gamma emission. Gamma carries energy but has A = 0 and Z = 0.
5. Exam Tips
- If asked “how many neutrons?”, write N = A-Z immediately.
- For decay equations: check conservation of both A and Z.
- After finding an unknown Z, check that the element symbol matches that proton number.
6. Worked Examples
Modelled example 1
Counting particles
Problem
Study the worked solution
Read nuclear counts
Method
Give 11 protons and calculate 12 neutrons.Reason
Z counts protons and A-Z counts neutrons.Working
p = 11, n = 23-11 = 12Apply neutrality
Method
Give 11 electrons.Reason
A neutral atom has equal positive and negative charge counts.Working
e = p = 11
Guided practice 2
Alpha decay equation
Problem
Balance A and Z independently
Hints
Hint 1: nucleon number
Hint 2: proton number
View solution step by step
Balance the two totals
Method
Subtract alpha values from the parent.Reason
Total A and total Z are conserved.Working
A = 222, Z = 86Write the daughter
Method
Identify Z = 86 as radon and complete the equation.Reason
The element symbol must match proton number.Working
²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He
Common misconception 3
Beta (β−) decay equation
Learner response
Make daughter plus particle equal the parent
View solution step by step
Balance A
Method
Keep daughter nucleon number at 14.Reason
The beta particle has A = 0.Working
14 = A_daughter + 0Balance Z
Method
Set daughter proton number to 7.Reason
7 + (-1) = 6, matching the parent.Working
6 = 7 + (-1)Identify the daughter
Method
Write nitrogen-14.Reason
Z = 7 identifies nitrogen.Working
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
Examiner practice 4
Identify the emitted particle
Examination question
Subtract daughter totals from parent totals
View solution step by step
Find particle numbers
2 marksMethod
Subtract lead’s A and Z from polonium’s.Reason
The emitted particle carries the missing conserved totals.Working
A = 210-206 = 4, Z = 84-82 = 2Identify it
1 markMethod
State alpha particle or helium-4 nucleus.Reason
A = 4, Z = 2 is ⁴₂He.Working
? = ⁴₂He
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark A difference, Z difference and identification.
Challenge 5
Spotting β− from A and Z
Inverse-equation transfer
Work backward from the nuclear-number changes
Hints
Hint 1: missing A
Hint 2: missing Z
View solution step by step
Infer particle values
Method
Assign the emitted particle A = 0 and Z = -1.Reason
Those values restore the parent’s totals.Working
Aₚ = A_d + 0, Zₚ = (Zₚ + 1) + (-1)Identify beta-minus
Method
State β⁻ emission.Reason
A beta-minus electron is written ⁰₋₁e.Working
β⁻ = ⁰₋₁e
7. Mind Stretchers
Mind stretcher 1: Chemical vs nuclear propertiesExtension
Why do isotopes usually have the same chemical properties but different nuclear properties?
Show Answer
Chemical properties depend mainly on the electron arrangement, which depends on the proton number Z. Nuclear properties depend on the nucleus (protons and neutrons), so changing neutrons can change stability.
Mind stretcher 2: Why conserve A and Z?Extension
Why do we balance nuclear equations by conserving total A and total Z?
Show Answer
A represents the total number of nucleons (protons + neutrons), so it is conserved. Total Z represents total charge (taking emitted particles into account), so it is conserved as well.
8. Practice and next step
Balance several A and Z totals in the Radioactivity Structured Practice, then continue to Radioactive Decay.
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Course and syllabus information
- Course
- SEC G3 Physics
- Edition
- SEC G3 Physics 2027