IB Physics SLTopic 5 — FissionPaper 1 & 2splitting a heavy nucleus~7 min read
Spontaneous & Induced Fission
Fire a slow neutron at a uranium-235 nucleus and it wobbles, stretches, and tears itself in two — releasing a burst of energy and a couple more neutrons in the process. That single split is the reaction behind every nuclear power station. Understanding how it starts, and why it releases so much energy, is where the whole Fission topic begins.
📘 What you need to know
Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei
The products are two daughter nuclei plus two or three neutrons, and gamma rays are also emitted
Isotopes of uranium and plutonium are used as fuels because they undergo fission readily
Energy is released because nuclear potential energy is transferred to the kinetic energy of the fragments, then carried away as heat
Spontaneous fission (a nucleus splitting on its own) is rare — most fission needs help
Induced fission is triggered when a nucleus absorbs a neutron, becoming unstable and splitting almost immediately
The absorbed neutron is slow-moving — a thermal neutron — and fission equations must balance nucleon (A) and proton (Z) numbers
What Is Fission?
Nuclear fission is the splitting of a large, unstable nucleus into two smaller ones. When it happens, a heavy nucleus like uranium-235 breaks into two daughter nuclei, throws out two or three neutrons, and emits gamma rays. Crucially, it also releases a large amount of energy — the reason uranium and plutonium are used as nuclear fuels.
A slow neutron is absorbed by uranium-235, which becomes unstable and splits into two daughter nuclei, releasing two or three fast neutrons and a large amount of energy as kinetic energy and gamma rays.
The fragments fly apart very fast. That’s because the nucleus’s stored nuclear potential energy is converted into the kinetic energy of the daughter nuclei and neutrons, which is then carried away as heat — the heat a reactor uses to make steam.
nuclear potential energy
→ converts to →
kinetic energy of fragments
→ carried away as →
heat
Spontaneous Fission
Occasionally a large nucleus splits entirely on its own, with no trigger — this is spontaneous fission. But it’s rare: for most heavy nuclei, splitting without any added energy almost never happens. Uranium-235, for instance, has a half-life of about 700 million years, so on its own it has very low activity. To get useful energy out, you need to make fission happen far more often — and that means inducing it.
Induced Fission
Almost all the fission we rely on is induced: it’s triggered deliberately. For fission to occur, the unstable nucleus must first absorb a neutron. This extra neutron makes the nucleus so unstable that it splits by fission almost immediately.
The neutron used has to be slow-moving — a thermal neutron. Take uranium-235: when it absorbs a neutron it briefly becomes uranium-236, which is highly unstable and splits at once. A fast neutron would simply bounce off, so the neutron must be slowed down first for the reaction to work.
U-235 + thermal neutron
→ forms →
U-236 (very unstable)
→ splits →
fission!
Balancing Fission Equations
Just like decay equations, fission equations must balance: the total nucleon number (A) and the total proton number (Z) are the same on both sides. This lets you find a missing product — usually the number of neutrons released.
Balancing rule
sum of A (left) = sum of A (right) & sum of Z (left) = sum of Z (right)
🧭 Filling in a missing fission product
Add up the nucleon numbers (A) on the left, and on the known part of the right
The difference is the missing A — often shared among neutrons (each neutron has A = 1)
Add up the proton numbers (Z) on both sides; the difference is the missing Z
A neutron has Z = 0, so if the missing Z is 0, the missing particles are neutrons
State the count — e.g. a missing A of 3 with Z of 0 means three neutrons
Quick recap: fission splits a heavy nucleus into two daughter nuclei plus 2–3 neutrons and energy; spontaneous fission is rare, so we use induced fission by firing a slow thermal neutron at U-235; and every fission equation must balance A and Z.
WE 1
A uranium-235 nucleus absorbs a neutron and splits: 23592U + 10n → 14156Ba + 9236Kr + ? Determine the missing product.
Step 1 — balance the nucleon numbers (A)
left: 235 + 1 = 236
right so far: 141 + 92 = 233
missing A = 236 − 233 = 3Step 2 — balance the proton numbers (Z)
left: 92 + 0 = 92
right so far: 56 + 36 = 92
missing Z = 92 − 92 = 0Step 3 — identify the particles
A = 3, Z = 0, and each neutron is 10n
the missing product is 3 neutrons (310n)
WE 2
(a) Explain the difference between spontaneous and induced fission. (b) Explain why the neutron used to induce fission must be slow-moving.
Part (a) — spontaneous vs induced
spontaneous: a nucleus splits on its own, with no trigger — this is rare
induced: a neutron is absorbed first, making the nucleus unstable so it splits
→ induced fission is deliberately triggered by adding a neutronPart (b) — why a slow neutron
a slow (thermal) neutron is more likely to be absorbed by the nucleus
a fast neutron would simply rebound off the nucleus without being captured
→ only a slow neutron is captured, so fission can occur
💡 Top tips
Fission always gives two daughter nuclei plus neutrons (usually 2 or 3) — and releases energy and gamma rays
Balance A and Z to find a missing product; a missing Z of 0 means the particles are neutrons
Induced fission needs a slow neutron. Say “thermal neutron” and note a fast one would just bounce off
The energy comes out as kinetic energy of the fast-moving fragments, then heat — not as light or sound
⚠ Common mistakes
Confusing fission with fusion — fission splits a heavy nucleus; fusion joins light ones
Saying induced fission needs a fast neutron — it needs a slow, thermal one to be absorbed
Forgetting the emitted neutrons when balancing — the missing product is usually 2 or 3 neutrons
Thinking spontaneous fission is common — it’s rare, which is why we induce fission with neutrons
Up next: Energy Released in Fission Reactions. You’ve seen how fission starts — next we calculate exactly how much energy each split releases, using binding energy per nucleon and comparing uranium’s energy density to everyday fuels.
Want this to actually click before the exam?
Book a free meeting and let’s work through the tricky bits together.
