6.18-6.22 - Nuclear decay and nuclear equations
Use changes in protons and neutrons to explain radioactive emissions, then balance nuclear equations one number row at a time. A gamma emission removes energy without changing the element.
Beta-minus and beta-plus decay
Radioactive decay changes an unstable nucleus. In the GCSE model for the two beta processes, one type of nucleon changes into the other and a charged beta particle is emitted from the nucleus.
In beta-minus decay:
neutron → proton + electron
The emitted electron is the beta-minus particle. It is produced in the nuclear change and emitted from the nucleus; it is not an electron that was already orbiting the nucleus. A neutron has become a proton, so the nucleus gains one proton but keeps the same total number of nucleons.
In beta-plus decay:
proton → neutron + positron
A positron has the same very small mass as an electron but a positive charge. A proton has become a neutron, so the nucleus loses one proton but keeps the same total number of nucleons.
| Process | Change inside the nucleus | Emitted particle | Change in mass number | Change in atomic number |
|---|---|---|---|---|
| Beta-minus | neutron becomes proton | electron, written beta- | 0 | +1 |
| Beta-plus | proton becomes neutron | positron, written beta+ | 0 | -1 |
The mass number is unchanged in both processes because a proton and a neutron each count as one nucleon. The atomic number changes because it counts protons only. The sign of the emitted particle is therefore not, by itself, the change to the daughter's atomic number: use the nucleon conversion or balance the whole equation.
These are the GCSE nucleon-and-charge bookkeeping equations. In a full physical description beta decay also emits a neutrino or antineutrino; that neutral particle changes neither of the two number balances used here.
How nuclear emissions change the numbers
Nuclear notation records two different counts:
- A is the mass number or nucleon number: protons plus neutrons.
- Z is the atomic number or proton number: protons only.
- X is the element symbol. If Z changes, the nucleus becomes a different element.
The number attached to an emitted particle tells you what leaves the parent nucleus. This gives the daughter-nucleus changes below.
| Emission | Symbol used in an equation | What leaves or changes | Daughter mass number A | Daughter atomic number Z |
|---|---|---|---|---|
| Alpha | two protons and two neutrons | decreases by 4 | decreases by 2 | |
| Beta-minus | neutron becomes proton; electron emitted | unchanged | increases by 1 | |
| Beta-plus | proton becomes neutron; positron emitted | unchanged | decreases by 1 | |
| Gamma | energy leaves as electromagnetic radiation | unchanged | unchanged | |
| Neutron | one neutron | decreases by 1 | unchanged |
The lower number on an emitted beta particle is its charge number for balancing; it is not a proton count inside the electron or positron. For example, in beta-minus decay the daughter atomic number rises by one, so adding the emitted electron's -1 returns the parent's bottom-number total.
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