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OCR GCSE Combined Science A · J250
OCR J250 · P4.3g Check the specification (PDF) (opens in a new tab)
A radioactive decay equation shows an unstable changing into a and emitting radiation. To complete the equation, you must account for both the nucleons and the electric charge.
A nucleus is written as . The upper number, , is its mass number: the total number of protons and neutrons. The lower number, , is its atomic number: the number of protons. Each proton carries one unit of positive charge, so also tells you the nucleus’s charge in these units.
Two checks are needed:
The upper-number check counts nucleons, rather than masses measured in kilograms. The lower-number check accounts for charge, including any negative charge carried away by an emitted electron.
An alpha particle contains two protons and two neutrons. Its mass number is therefore 4 and its charge is +2. It can be written as or as a helium nucleus, .
When a nucleus emits an alpha particle, the daughter has four fewer nucleons and two fewer protons. For example, polonium-212 decays into lead-208:
Check the upper numbers: . Check the lower numbers: . Both balance. The daughter is a different element because its proton number has changed.
If the daughter’s numbers were missing, you would find them by subtraction: and .
In beta-minus decay, a neutron in the nucleus changes into a proton and an electron. The electron is emitted as the beta particle. There is still the same total number of protons and neutrons, but there is now one extra proton.
The emitted electron is written as . Its upper number is 0 because it contains no protons or neutrons; its lower number is −1 because it carries one unit of negative charge.
Carbon-14 provides an example:
The upper numbers balance because . The lower numbers balance because . The daughter’s positive charge increases, but the emitted electron carries away negative charge, so the total charge stays the same.
When finding a missing daughter atomic number, remember that it must satisfy . Therefore , not 5.
Gamma radiation carries energy away from the nucleus but contains no nucleons and carries no electric charge. It is represented in a balanced equation by .
For example, an excited uranium-238 nucleus can lose energy by emitting gamma radiation. The star marks the higher-energy nucleus:
The checks are and . The daughter is still uranium-238: its energy has decreased, but its proton and neutron numbers have not changed.
Start by identifying the radiation and writing its two numbers. Then find the missing upper number using the mass-number balance and the missing lower number using the charge balance. Finally, add the numbers on the right to check that they match those on the left.
You can also work backwards to identify missing radiation. A difference of 4 in mass number and 2 in atomic number indicates an alpha particle. An emitted particle with numbers 0 and −1 is a beta-minus particle. Gamma emission leaves both numbers unchanged.
For , counts protons + neutrons; counts protons.
| Emission | Emitted radiation | Daughter mass number | Daughter atomic number |
|---|---|---|---|
| Alpha |
Alpha and beta-minus decay change the element. Gamma emission lowers the nucleus’s energy without changing its proton or neutron numbers.
Write the radiation’s numbers → calculate the missing upper and lower numbers separately → check both totals.
Check the upper numbers and the lower numbers separately: both must balance across the arrow.
For beta-minus emission, include the electron’s lower number of −1. The daughter’s atomic number increases by 1, rather than decreasing.
An alpha particle may be written as or : both represent two protons and two neutrons.
A beta particle’s mass number of 0 does not mean that an electron has no mass; it means that it contains no nucleons.
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Mass number
The total number of protons and neutrons in a nucleus, written as the upper number in nuclear notation.
Atomic number
The number of protons in a nucleus, written as the lower number in nuclear notation. It determines the element and the nucleus’s positive charge.
Parent nucleus
The original unstable nucleus that undergoes radioactive decay.
Daughter nucleus
The nucleus remaining after radioactive decay.
Alpha particle
A particle consisting of two protons and two neutrons, represented by or .
Beta-minus particle
An electron emitted from a nucleus when a neutron changes into a proton, represented by .
Gamma radiation
Electromagnetic radiation emitted when a nucleus loses energy, with no change in its mass number or atomic number.
Put your knowledge into practice — try past paper questions for Combined Science A
Mass number
The total number of protons and neutrons in a nucleus, written as the upper number in nuclear notation.
Atomic number
The number of protons in a nucleus, written as the lower number in nuclear notation. It determines the element and the nucleus’s positive charge.
Parent nucleus
The original unstable nucleus that undergoes radioactive decay.
Daughter nucleus
The nucleus remaining after radioactive decay.
Alpha particle
A particle consisting of two protons and two neutrons, represented by or .
Beta-minus particle
An electron emitted from a nucleus when a neutron changes into a proton, represented by .
Gamma radiation
Electromagnetic radiation emitted when a nucleus loses energy, with no change in its mass number or atomic number.
| Beta-minus |
| Gamma |