Section 1 of 5Reading a Fission Diagram
A fission diagram shows a large nucleus splitting into two smaller nuclei and releasing neutrons and energy.
Parent nucleus
- Parent nucleus A large nucleus such as uranium-235 absorbs a slow-moving neutron and becomes unstable.
Daughter nuclei
- Daughter nuclei The unstable nucleus splits into two smaller daughter nuclei of roughly similar size.
Neutrons
- Neutrons Two or three fast-moving neutrons are released, and these can go on to cause further fissions.
Energy
- Energy The kinetic energy of the daughter nuclei and neutrons is shared with surrounding particles and transferred as thermal energy.
Mass loss
- Mass loss The total mass of the products is slightly less than that of the reactants, and the missing mass is released as energy.
Section 2 of 5Writing a Fission Equation
Equations for nuclear reactions must balance both mass number and proton number.
1
Write the parent nucleus and the incoming neutron on the left, each with its mass number above and proton number below.
2
Choose the two daughter nuclei and write their symbols on the right, with mass and proton numbers.
3
Add the mass numbers on each side and check that the totals are equal.
4
Add the proton numbers on each side and check that the totals are equal, which confirms charge is conserved.
5
Count any leftover neutrons, which have mass number 1 and proton number 0, and add them to the right-hand side.
| Quantity | Before fission | After fission |
|---|---|---|
| Mass number (nucleon number) | 235 + 1 = 236 | 90 + 143 + 3 = 236 |
| Proton number | 92 + 0 = 92 | 38 + 54 + 0 = 92 |
Section 3 of 5Chain Reactions
Neutrons released by one fission can trigger further fissions, and this can be controlled or uncontrolled.
Chain Reactions
Chain reaction
- Chain reaction Neutrons released in one fission cause further fissions, which can build up rapidly if nothing is done to limit them.
Controlled reactor
- Controlled reactor Control rods absorb neutrons so that, on average, only one neutron from each fission causes another fission.
Critical mass
- Critical mass A minimum amount of fissile material is needed for a chain reaction to be self-sustaining.
Uncontrolled reaction
- Uncontrolled reaction If the number of fissions grows without control, the energy release rises very quickly, which is dangerous.
Section 4 of 5Worked Example
Question Complete the nuclear equation ²³⁵₉₂U + ¹₀n → ⁹⁰₃₈Sr + ¹⁴³₅₄Xe + x ¹₀n by finding the value of x.
- Total mass number on the left = 235 + 1 = 236.
- Total mass number of the two daughter nuclei = 90 + 143 = 233.
- Missing neutrons236 − 233 = 3, so x = 3.
- Proton checkleft side 92 + 0 = 92, right side 38 + 54 = 92, so the equation balances.
Answer x = 3, so three neutrons are released in this fission.
Section 5 of 5Exam Checklist & Tips
4.4.3 – Key Points for the Exam
- Nuclear fission The splitting of a large, unstable nucleus into two smaller nuclei, with the release of neutrons and energy.
- Daughter nuclei The nuclei produced when a parent nucleus undergoes fission.
- Neutron An uncharged particle in the nucleus with a mass number of 1.
- Chain reaction A reaction in which the products of one step cause the next step to occur.
- Critical mass The minimum mass of fissile material needed to sustain a chain reaction.
- Mass number The total number of protons and neutrons in a nucleus.
Common Mistakes
- Forgetting the neutron on the left-hand side when adding up the mass numbers.
- Balancing the mass numbers but not the proton numbers, which breaks the conservation of charge.
- Thinking fission happens mainly in stars;
- fission happens in heavy nuclei, while fusion is the process that powers stars.
- Assuming the two daughter nuclei are always the same; the products vary from one fission to the next.