Before you can read the parent note, you need to earn every symbol it throws at you. This page walks each one from absolute zero: what it means in plain words, the picture it corresponds to, and why the topic can't do without it.
Everything begins with a picture of what sits at the centre of an atom.
Why does the topic need this? Because fission is literally the rearrangement of these two ingredients — a big ball of them breaks into two smaller balls, and a few loose neutrons fly off. If you don't picture the nucleons, none of the arrows in the parent note mean anything.
The parent note is full of things like 92235U. This is not scary once you know what each number counts.
Why the topic needs it: fission equations must balance these numbers. In the parent's reaction the top numbers add to 236 on both sides, and the bottom numbers add to 92 on both sides. Those two "must-balance" rules are conservation of nucleons and charge — you can't check a fission equation without reading A and Z.
235U and 238U are both uranium (both have 92 protons), but one has 143 neutrons and the other 146. The topic treats them as completely different characters:
235U = fissile — splits when it swallows a slow neutron.
238U = fertile — usually just absorbs a neutron and later turns into plutonium.
Why the topic needs it: the whole "enrichment" discussion (3–5% vs 15–20%) is about the ratio of these two isotopes in the fuel. Without the isotope idea, "enrichment" is a meaningless word.
To understand why a nucleus is "barely holding together" you need the tug-of-war picture.
Why the topic needs it: this balance explains why heavy nuclei fission but light ones don't, and why a single extra neutron is enough to trigger the split.
This is the single most important idea feeding the whole topic, so we build it carefully.
Figure s04 is the curve of the whole subject. Read it left to right:
It rises steeply for light nuclei, reaches a peak near iron (A≈56, about 8.8 MeV/nucleon), then gently falls for heavy nuclei.
235U sits on the right-hand downslope, at only ≈7.6 MeV/nucleon — the top of the hill is above it.
When 235U splits into two mid-sized fragments (A≈117), those fragments sit higher on the curve (≈8.5 MeV/nucleon) — more tightly bound.
For the deeper machinery of where binding energy comes from (mass turning into energy), see Binding Energy & Mass Defect. The mirror-image process — light nuclei climbing the left slope by joining — is Nuclear Fusion.
The parent note claims "mass is not conserved" in fission. Here's the plain-words version.
Why the topic needs it: this is how the parent turns a mass difference into "200 MeV." The full story lives in Einstein Mass-Energy Equivalence.
Recall What is an MeV, in one line?
Q: What does "MeV" measure and why so tiny a unit?
A: A mega-electron-volt is a unit of energy sized for single particles — nuclear events involve one atom at a time, so joules would be absurdly small decimals.
Once one fission happens, 2–3 neutrons fly out. The question that runs the whole topic is: how many of them cause the NEXT fission?
Whether a neutron gets captured or leaks depends on how likely it is to hit a nucleus — measured by the Neutron Cross-section. That's why "slow vs fast neutrons" matters so much in the parent note.
The parent explains critical mass with "surface vs volume." Here is the geometry from zero.
Here r = the radius (how far from centre to edge). A sphere is chosen because, for a given amount of material, it has the smallest possible skin — the least leakage.
Three job-titles you must know before the reactor section:
The related ideas of neutron behaviour over time, and what happens to leftover radioactive fragments, connect to Radioactive Decay & Half-life and Nuclear Reactor Safety & Waste.
Read it top-down: the two ingredients let you write nuclide symbols, which give isotopes; the force tug-of-war explains binding energy, which (through mass defect) gives the released energy; that energy plus the emitted neutrons drives k, and k governs both critical mass and reactor control — the whole parent topic.
