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.
What a learner can do afterwards
- 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
1 · Read
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.
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.
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.
Read the neutron-proton balance to name the decay, balance mass and charge, and price the missing mass as energy.
More to read
21.3 Radioactive Decay
Unstable nuclei relax toward the stable band by emitting particles. Neutron rich isotopes convert a neutron to a proton and eject an electron, while proton rich ones emit a positron or capture an atomic electron. Heavyweights above lead often throw off a whole alpha particle to cut bulk fast. The neutron to proton ratio is the quickest predictor of which route a nucleus takes.
31.4 Nuclear Decay and Conservation Laws
Every nuclear equation must balance total nucleons and total charge. Alpha decay drops mass number by four and atomic number by two; beta minus raises atomic number by one with mass unchanged. Gamma emission alone changes neither, draining only excitation energy. Apply both tallies to any proposed reaction and mistakes surface at once.
31.5 Half-Life and Activity
Half-life turns randomness into a schedule: after each half-life, half the remaining nuclei persist. Activity measures the decay rate in becquerels, decays per second, and it falls with the same clock. Given the starting sample and the half-life, you can forecast the survivors after any number of periods. Archaeologists and doctors use the same math for dating and dosing.
10.4 Nuclear Reactions
Nuclear reactions generalize decay: particles collide, rearrange, and release the mass difference as energy. Write reactants and products, balance nucleons and charge, then compare the masses. The Q value, initial mass minus final mass times c squared, tells you the energy out. Positive Q means the reaction can run on its own books.
Explore more
2 · Watch
En anglaisTake it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.