Nuclear Fission and Fusion
Nuclear fission is the splitting of a heavy nucleus into two medium-mass nuclei with release of energy and usually neutrons. Nuclear fusion is the combination of light nuclei to form a heavier nucleus with release of energy under extremely high temperature and pressure conditions.
In one line: Fission splits heavy nuclei; fusion joins light nuclei; both pay out energy by moving toward stronger binding.
Concept Practice Coming SoonUnderstand it
- Energy is released in both processes because the products have greater binding energy per nucleon than the reactants.
- In fission, a heavy nucleus such as uranium-235 may absorb a neutron and split into fragments, releasing more neutrons that can continue a chain reaction.
- In fusion, light nuclei such as hydrogen isotopes combine, but high temperature is needed to overcome electrostatic repulsion between positively charged nuclei.
- The binding energy curve explains both processes as movement toward more stable nuclei near the peak region.
Example
Formula and rule
Energy released Q = (mass of reactants - mass of products)c2 when the product mass is smaller.
In nuclear units, Q in MeV = mass difference in u x 931.5.
Fission is favoured for very heavy nuclei, while fusion is favoured for light nuclei because both can move products toward higher binding energy per nucleon.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
If a nuclear reaction has total reactant mass 235.124 u and total product mass 234.904 u, mass difference = 0.220 u.
- 1Energy released = 0.220 × 931.5 MeV = 204.93 MeV.
- 2Since product mass is smaller, the reaction releases about 205 MeV of energy.
Method you can reuse
- 1Identify whether nuclei are heavy or light.
- 2For fission, describe splitting, emitted neutrons, and possible chain reaction.
- 3For fusion, describe combination of light nuclei and need for high temperature.
- 4Use binding energy per nucleon to explain energy release.
- 5For numericals, calculate mass difference and convert to MeV.
Visual tip
A comparison diagram may show a heavy nucleus splitting into two fragments and neutrons for fission, and two light nuclei combining for fusion. If linked to the binding energy graph, mark heavy nuclei moving left toward higher binding energy per nucleon and light nuclei moving right toward higher binding energy per nucleon.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
Why can both fission and fusion release nuclear energy?
Think first, then show answerHide answer
Common wrong answer
Fusion occurs easily at room temperature because it releases energy.
Why it loses marks: Fusion needs extremely high temperature because positively charged nuclei must come close enough for nuclear force to act.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
- 1
Enough for 1 mark
Nuclear fission is the splitting of a heavy nucleus into two medium-mass nuclei with release of energy and usually neutrons. Nuclear fusion is the combination of light nuclei to form a heavier nucleus with release of energy under extremely high temperature and pressure conditions.
- 2
Add for 2 marks
Example: In a fission chain reaction, one neutron absorbed by uranium-235 can produce fission fragments, energy, and additional neutrons. In the Sun, fusion of light nuclei is responsible for the large energy output.
- 3
Add for 3 marks
Energy is released in both processes because the products have greater binding energy per nucleon than the reactants.
Concept practice is coming soon
Login to access concept-level MCQs and weak-concept practice as they become available.
Help improve this page
Found something confusing, incorrect, or missing?