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2.9.4Forces, Space and Radioactivity

Nuclear Equations for Fission and Fusion

5 min read 5 parts Lesson 41 of 41 Unit quiz

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Section 1 of 5Reading Nuclear Notation

2.9.4 Nuclear Equations for Fission and Fusion

A nuclide is written with its nucleon number at the top and its proton number at the bottom, as in ²³⁵₉₂U.

Nucleon number (A)
  • The top number counts protons and neutrons in the nucleus together.
Proton number (Z)
  • The bottom number counts the protons only, and it identifies the element.
Neutron
  • A neutron is written ¹₀n, with nucleon number 1 and proton number 0.
Balancing rule
  • In any nuclear equation the totals of the top numbers and the totals of the bottom numbers must be equal on both sides.

Section 2 of 5A Fission Equation

Uranium-235 absorbs a neutron and splits into krypton, barium and three neutrons.

1
Write the equation: ²³⁵₉₂U + ¹₀n → ⁹²₃₆Kr + ¹⁴¹₅₆Ba + 3 ¹₀n + energy.
2
Check nucleon numbers: 235 + 1 = 236 on the left and 92 + 141 + 3 = 236 on the right.
3
Check proton numbers: 92 + 0 = 92 on the left and 36 + 56 + 0 = 92 on the right.
4
The energy is released as kinetic energy of the products and as gamma radiation.
5
Many different pairs of daughter nuclei can form, so you must work out the missing values each time.

Section 3 of 5A Fusion Equation

Deuterium and tritium fuse to give helium and a neutron.

A Fusion Equation
Equation
  • ²₁H + ³₁H → ⁴₂He + ¹₀n + energy.
Nucleon numbers
  • 2 + 3 = 5 on the left and 4 + 1 = 5 on the right.
Proton numbers
  • 1 + 1 = 2 on the left and 2 + 0 = 2 on the right.
Energy
  • The products have a slightly smaller total mass than the reactants, and this difference is released as energy.

Section 4 of 5Worked Example

Question

  • Uranium-235 absorbs a neutron and splits into barium-141, a krypton nucleus and three neutrons.
  • Find the nucleon number and proton number of the krypton nucleus.

  1. Write the equation²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ᴬ_ZKr + 3 ¹₀n.
  2. Nucleon numbers235 + 1 = 141 + A + 3, so A = 236 − 144 = 92.
  3. Proton numbers92 + 0 = 56 + Z + 0, so Z = 92 − 56 = 36.

Answer Krypton-92: nucleon number 92 and proton number 36, written ⁹²₃₆Kr

Section 5 of 5Exam Checklist & Tips

2.9.4 – Key Points for the Exam

  • Nucleon number The total number of protons and neutrons in a nucleus.
  • Proton number The number of protons in a nucleus, which defines the element.
  • Daughter nucleus A nucleus formed as a product of fission.
  • Nuclear equation An equation showing the nuclei that react and the particles produced, balanced in A and Z.

Common Mistakes

  • Forgetting to count the neutron that starts a fission reaction on the left-hand side.
  • Ignoring the neutrons on the right-hand side when balancing the nucleon number.
  • Mixing up which number is the nucleon number and which is the proton number.
  • Changing the proton number of the neutron, which should be zero because it has no charge.
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