--- title: "Decay Modes, Decay Chains and Nuclear Reactions" description: "Which decay a nucleus takes depends on where it sits relative to the line of stability, and a heavy nucleus usually needs a chain of steps to reach it. Reaction equations balance charge, nucleon numbe" canonical: https://lightmysky.com/learn/science/decay-modes-decay-chains-and-nuclear-reactions-mt_b_imidwJeQ source: https://lightmysky.com/learn/science/decay-modes-decay-chains-and-nuclear-reactions-mt_b_imidwJeQ.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Decay Modes, Decay Chains and Nuclear Reactions Which decay a nucleus takes depends on where it sits relative to the line of stability, and a heavy nucleus usually needs a chain of steps to reach it. Reaction equations balance charge, nucleon number and energy. Subject: Science · Area: Quantum & Modern Physics · Ages 20 to 21 Page: https://lightmysky.com/learn/science/decay-modes-decay-chains-and-nuclear-reactions-mt_b_imidwJeQ ## Ready when they can - Predicts the likely decay mode of a nucleus from its neutron to proton ratio - Balances a nuclear reaction and computes its energy release from the mass difference - Follows a decay chain to a stable end point ## Lesson: Which decay a nucleus chooses Find your nucleus against the band of stability. Neutron-heavy nuclei above the band turn a neutron into a proton and spit out an electron: beta minus. Proton-heavy nuclei below the band turn a proton into a neutron, either emitting a positron or capturing an inner electron. Very heavy nuclei above mass 200 also shed alpha particles. Balance every equation twice: total mass number on each side, total charge on each side. Uranium-238 sheds an alpha to become thorium-234, losing 4 in mass and 2 in charge. That thorium is still restless and beta decays with a 24-day half-life into protactinium-234, gaining one proton with no mass change; the electron is created inside the nucleus at that moment, not pulled from orbit. **Example.** Weigh both sides: the missing mass times c squared is the released energy, so plutonium-239 alpha decay frees 5.25 MeV, millions of times a chemical reaction. A nucleus left excited can also relax by gamma emission, as cobalt-60 does, with no change in mass or charge at all. **Tip.** Follow each daughter to its own decay until you land on something stable, often lead. The pace is set by the half-life, and activity counts decays per second in becquerels. Positron emitters like fluorine-18 even serve medicine: each positron soon meets an electron and the pair's gamma burst maps the body in a PET scan. **Recap.** Read the neutron-proton balance to name the decay, balance mass and charge, and price the missing mass as energy. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Properties of Nuclei and the Nuclear Force](https://lightmysky.com/learn/science/properties-of-nuclei-and-the-nuclear-force-mt_HdMS_muGWa) - [The Decay Constant and the Exponential Decay Law](https://lightmysky.com/learn/science/the-decay-constant-and-the-exponential-decay-law-mt_VIydSAys1W) ## Opens up - [Fission Reactors and Fusion in the Laboratory](https://lightmysky.com/learn/science/fission-reactors-and-fusion-in-the-laboratory-mt_iTwgXI0iBy)