Write and check alpha decay equations
Conserve nucleon number and charge number while identifying parent daughter and alpha particle.
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Read nuclide notation.
- Distinguish nucleon counts from exact rest mass.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which daughter change is correct for alpha emission?
What you will learn
- Conserve nucleon number and charge number while identifying parent daughter and alpha particle.
- Write alpha equations with balanced number rows and correctly identified daughter elements.
Balance the nuclide labels
An alpha particle is ⁴₂He. In alpha decay the daughter has nucleon number A − 4 and proton number Z − 2. Neutron number also falls by two. For example, ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He balances 226 = 222 + 4 and 88 = 86 + 2.
The daughter symbol must match its proton number. A label with the right nucleon number but wrong element is not a correct equation. Supply or consult the stated element mapping rather than guessing a symbol from the parent.
Interpret what is conserved
The sums of nucleon numbers and charge numbers balance in these equations. Integer nucleon number is not an exact rest-mass value, so the balance does not imply that nuclear energy release is zero. The equation identifies the nuclear change without calculating an energy value.
The daughter may still be radioactive. In the supplied radium example, writing one balanced step does not claim that radon is stable or that all later emissions have ended.
Sources for this lesson
Pause and explain
Does a balanced alpha equation prove zero released energy?
Worked example
Complete ²²⁶₈₈Ra → ? + ⁴₂He, given Z = 86 identifies Rn.
Show the worked solution
- Subtract 4 from 226: 222.
- Subtract 2 from 88: 86.
- Use the supplied daughter symbol Rn.
Answer ²²²₈₆Rn.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Complete the radium alpha equation with supplied daughter element Rn.
Give me a hint
Subtract the alpha labels from the parent labels.
Compare my reasoning
- 226 − 4 = 222.
- 88 − 2 = 86.
- The daughter is ²²²₈₆Rn.
²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He.
Look for these in your work
- Balance A.
- Balance Z.
²³⁸₉₂U emits alpha. Given Z = 90 is Th, write the daughter.
Give me a hint
Subtract four nucleons and two protons.
Compare my reasoning
- 238 − 4 = 234.
- 92 − 2 = 90.
- Use Th for proton number 90.
²³⁴₉₀Th.
Look for these in your work
- Keep the symbol consistent.
- Do not use unchanged uranium.
A proposed equation is ²²⁶₈₈Ra → ²²⁴₈₆Rn + ⁴₂He. Find and correct the error.
Give me a hint
Check the total nucleon number.
Compare my reasoning
- The right A total is 228.
- The left A is 226.
- Change the daughter A to 222.
Replace ²²⁴₈₆Rn with ²²²₈₆Rn.
Look for these in your work
- Show the failed total.
- Retain the already-correct Z balance.
Common mistakes
- Check both number rows.
- Alpha is a helium nucleus.
- A balanced step does not prove stability.
What can you explain now?
How many nucleons are in alpha?
Compare with the explanation
Four
Alpha contains two protons and two neutrons.
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Make it stick
Write alpha equations with balanced number rows and correctly identified daughter elements.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.