06.08B Nuclear reactions and energy assignment

Module 06 โ€” Heat and Particles ยท quiz worth 25 pts

The big idea Nuclear processes rearrange or transform nuclei. Fission splits a heavy nucleus, fusion joins light nuclei through nuclear reactions, and radioactive decays change A and Z in predictable ways.

Balance A and Z

You can identify most assigned nuclear processes by tracking two numbers.

Not sure about the basics underneath this lesson?

Quick nuclear-change map

ProcessChange in AChange in ZWhat leaves or combines
Alpha decay โˆ’4 โˆ’2 Helium-4 nucleus
Beta-minus decay 0 +1 Electron and electron antineutrino
Gamma emission 0 0 Gamma photon
Fission Fragments sum with emitted neutrons Charge balances Two main fragments, neutrons, energy
Fusion Inputs and products balance Charge balances Light nuclei combine through nuclear reactions
Before you start Review atomic number Z and mass number A from 06.05. You will use conservation and signed changes to check every nuclear model.

Nuclear change is not chemical change

Chemical reactions rearrange electrons and bonds while nuclei retain their identities. Nuclear reactions or decays can change a nucleus's proton or neutron count. The released energy per event can be much larger because nuclear binding energies and mass-energy conversion are involved. The accounting still obeys conservation of total energy, momentum, charge, and nucleon number where applicable.

Fission and fusion

Fission splits a heavy nucleus into smaller nuclei, usually with free neutrons and energy. In induced fission, a fissile nucleus commonly captures a neutron; the resulting excited nucleus becomes unstable and splits. Released neutrons can sustain a controlled or uncontrolled chain reaction. Fusion combines light nuclei through a sequence of nuclear reactions. Because positively charged nuclei repel each other, fusion requires sufficiently energetic nuclei to approach closely enough for the strong nuclear interaction to act; high temperature supplies kinetic energy, while density, confinement, and quantum tunneling affect the reaction rate. In the Sun, the net proton-proton chain converts four protons into a helium-4 nucleus plus other products and energy. This is nuclear fusion, not chemical burning.

Stacked panels show a generic neutron-induced fission with two fragments, three neutrons, released energy, and balanced A and Z accounting, then generic fusion of two light nuclei with released energy.
Fission splits a heavy nucleus; fusion joins light nuclei. The generic relations keep charge and mass number balanced without inventing a specific uranium equation.

Decay bookkeeping

Alpha decay
ZAXโ†’Zโˆ’2Aโˆ’4Y+24He{}^{A}_{Z}X\rightarrow{}^{A-4}_{Z-2}Y+{}^{4}_{2}\text{He}

The emitted alpha particle contains two protons and two neutrons. Alpha decay is distinct from fission.

Beta-minus decay
nโ†’p+eโˆ’+ฮฝห‰en\rightarrow p+e^-+\bar{\nu}_e

For the nucleus, A stays the same and Z increases by 1.

Gamma emission
ZAXโˆ—โ†’ZAX+ฮณ{}^{A}_{Z}X^*\rightarrow{}^{A}_{Z}X+\gamma

A and Z stay the same while an excited nucleus loses energy.

Mass-energy release
E=ฮ”mc2E=\Delta mc^2

ฮ”m is the decrease in total rest mass between initial and final systems.

Track a nuclear equation

Write A above Z

A counts nucleons; Z counts protons and identifies the element.

Balance A

The total mass-number count must match across the reaction notation.

Balance Z or charge

The total proton-number or electric-charge accounting must match, including emitted particles.

Name the process from the change

A โˆ’4 and Z โˆ’2 signals alpha decay. Same A and Z +1 signals beta-minus. Unchanged A and Z with a photon signals gamma emission.

State the energy source

Use a binding-energy or mass-difference explanation, not chemical combustion language.

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Worked nuclear examples

Alpha decay of uranium-238

  • Parent. 92238U{}^{238}_{92}\text{U}
  • Emission. 24He{}^{4}_{2}\text{He}

Find the daughter A and Z.

  1. Subtract the alpha particle counts.
    A=238โˆ’4=234,Z=92โˆ’2=90A=238-4=234,\quad Z=92-2=90
  2. Atomic number 90 is thorium.
    92238Uโ†’90234Th+24He{}^{238}_{92}\text{U}\rightarrow{}^{234}_{90}\text{Th}+{}^{4}_{2}\text{He}

The daughter is thorium-234.

Mass converted to released energy

  • Mass decrease. ฮ”m=1.0ร—10โˆ’12โ€‰kg\Delta m=1.0\times10^{-12}\,\text{kg}
  • Light speed. c=3.00ร—108โ€‰m/sc=3.00\times10^8\,\text{m/s}

Find the energy equivalent.

  1. Use E = ฮ”mcยฒ.
    E=(1.0ร—10โˆ’12)(3.00ร—108)2โ€‰JE=(1.0\times10^{-12})(3.00\times10^8)^2\,\text{J}

E = 9.0 ร— 10โด J.

Build the five process models

Model each assigned process

Create a clear physical model or diagram for fission, fusion, alpha decay, beta decay, and gamma emission.

Show before and after

Name the starting and ending element or nucleus and label proton and neutron counts where the assignment asks.

Show what leaves

Label emitted particles or radiation and include energy in the model.

Explain each model

Describe what changed in the nucleus and how A and Z confirm the process.

Compare startup conditions

State that induced fission commonly begins with neutron capture, while fusion needs nuclei energetic enough to approach despite electrostatic repulsion.

Compare scales

Finish the requested nuclear-versus-chemical energy comparison without calling fusion burning.

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Nuclear-model checklist

  1. All five assigned processes are present.
  2. Every nucleus has a readable identity or A/Z label.
  3. Before-and-after proton and neutron counts agree with the notation.
  4. Emitted particles or photons are labeled.
  5. Energy is shown and explained using nuclear binding or mass difference.
  6. The startup comparison says induced fission commonly begins with neutron capture, while fusion requires sufficiently energetic nuclei to approach despite electrostatic repulsion.
  7. Alpha decay is not labeled fission.
  8. Fusion is not labeled chemical burning.
  9. The finished file or photos are legible before upload.

Nuclear-physics traps

Alpha decay is not fission

Alpha decay emits one helium-4 nucleus. Fission splits a heavy nucleus into two substantial fragments, usually with neutrons.

Fusion is not fire

Stellar fusion is a set of nuclear reactions; chemical burning rearranges electrons.

Gamma does not change the isotope

Gamma emission lowers nuclear excitation energy while A and Z remain unchanged.

Heavy does not automatically mean radioactive

Stability depends on the particular nuclide, not a simple rule that every element above iron is unstable.

Nuclear reactions practice

Balance A and Z before naming the process.