5.2.3Nuclear Physics
Radioactive Decay — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 11 parts Lesson 13 of 17 1 quiz
Curriculum Equation Key Radioactive Decay & Half-Life FormulasFast Exam Solution
N(t) = N₀ × (1/2)t / t½ | λ = ln(2) / t½ ≈ 0.693 / t½ | E = Δm × c²
Lesson summary
5.2.3 Radioactive Decay — IGCSE PHYSICS notes: Explain that radioactive decay is random and spontaneous, describe how alpha, beta and gamma emission change the nucleus and write balanced decay equations.Radioactive decay = the change in an unstable nucleus that emits alpha, beta …
- Too many neutrons
- Very heavy nuclei
- Neutron–proton balance
- Occurrence
- Parent nucleus
- Daughter nucleus
Part 1 of 9Radioactive decay
Radioactive decay
Radioactive decay the change in an unstable nucleus that emits alpha, beta or gamma radiation.
Spontaneous
- Spontaneous it is not affected by temperature, pressure or chemical state.
Random
- Random it is impossible to predict which nucleus will decay or when.
Chain
- Chain decay continues through a series of nuclei until a stable nucleus is reached.
Part 2 of 9Why nuclei are unstable (Extended)
Too many neutrons
- Too many neutrons isotopes with too many neutrons compared with protons tend to undergo beta decay, which reduces the neutron:proton ratio.
Very heavy nuclei
- Very heavy nuclei the forces holding them together are weaker, so they tend to undergo alpha decay to become lighter and more stable.
Neutron–proton balance
- Neutron–proton balance stable light nuclei have about as many neutrons as protons, while heavy nuclei need more.
Hydrogen
- Hydrogen hydrogen-1 is stable, and tritium is unstable.
- (2 neutrons to 1 proton)
Part 3 of 9Alpha decay
Change
- Change the nucleus loses 2 protons and 2 neutrons.
Numbers
- Numbers the nucleon number A decreases by 4 and the proton number Z decreases by 2.
Result
- Result a different element forms.
- (the daughter nucleus)
Part 4 of 9Beta decay
Change
- Change a neutron changes into a proton and an electron, and the electron is emitted.
Numbers
- A is unchanged and Z increases by 1.
Result
- Result a different element forms.
Part 5 of 9Gamma emission
Change
- Change the nucleus loses excess energy.
Numbers
- A and Z are unchanged and it is the same element.
Occurrence
- Occurrence gamma emission often follows alpha or beta decay.
Part 6 of 9Decay equations (Extended)
Alpha
- AZX → A−4Z−2Y + 42He.
Beta
- AZX → AZ+1Y + 0−1e.
Gamma
- AZX → AZX + γ.
Balancing
- Balancing the totals of nucleon numbers and of proton numbers are equal on both sides.
Notation
- Notation beta is 0−1e, alpha is 42He and gamma is 00γ.
Part 7 of 9Writing a decay equation
1
Write the parent nucleus in nuclide notation.
2
Identify the emission and write its symbol.
3
Subtract so that the nucleon numbers balance to find A of the daughter.
4
Balance the proton numbers to find Z of the daughter.
5
Use Z to name the new element and check the totals on both sides.
Part 8 of 9Effect on the nucleus
| Decay | Emitted | Change in A | Change in Z |
|---|---|---|---|
| Alpha | 42He | −4 | −2 |
| Beta | 0−1e (neutron to proton) | 0 | +1 |
| Gamma | γ | 0 | 0 |
Part 9 of 9More from the specification
More from the specification
Alpha and beta particle deflection in magnetic fields
- Alpha and beta particle deflection in magnetic fields perpendicular magnetic fields exert Lorentz forces deflecting charged emissions, providing a means of sorting.
Radioactive decay
- Radioactive decay random emission of ionising particles from an unstable atomic nucleus
Half-life
- Half-life time for the activity or number of undecayed nuclei of a source to fall to half
Background count
- Background count radiation from rocks, air and cosmic rays that must be subtracted from a source's count rate
Alpha, beta and gamma
- Alpha, beta and gamma alpha is a helium nucleus with positive charge and strongest ionisation, beta is a fast electron, gamma is electromagnetic radiation with the weakest ionisation
Radioactive decay
- Radioactive decay random emission of ionising particles from an unstable atomic nucleus
Half-life
- Half-life time for the activity or number of undecayed nuclei of a source to fall to half
Background count
- Background count radiation from rocks, air and cosmic rays that must be subtracted from a source's count rate
Alpha, beta and gamma
- Alpha, beta and gamma alpha is a helium nucleus with positive charge and strongest ionisation, beta is a fast electron, gamma is electromagnetic radiation with the weakest ionisation
Background radiation
- Radiation that is always present in the environment.
Penetrating power
- How far radiation travels through a material.
Ionising power
- How strongly radiation ionises atoms.
Electric field
- Region of space in which a charge experiences a force.
Magnetic field
- Area around a magnet where its effects can be detected.
Kinetic energy
- K = ½mv2, energy of motion.
Half-life
- Time for half the nuclei of an isotope to decay.
Source
- Reservoir at TH providing heat QH
Charge
- Quantity of electricity in coulombs, Q = It.What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
Detector
- An instrument that records the particles produced in collisions.
Key terms
| Term | Meaning |
|---|---|
| Radioactive decay | Spontaneous random change in an unstable nucleus that emits radiation. |
| Parent nucleus | The nucleus before decay. |
| Daughter nucleus | The nucleus formed by decay. |
| Alpha decay | Loss of 2 protons and 2 neutrons. |
| Beta decay | A neutron changes into a proton and an electron. |
Avoid these mistakes
1
Saying temperature or chemical state affects decay — it does not.
2
Using +1 for the nucleon number in beta decay — A stays the same.
3
Saying gamma emission forms a new element — it does not.
4
Not balancing both the top and bottom numbers in a decay equation.
Marked practiceQuizzes for this lesson
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