Lesson 7 of 9
Balance nuclear processes
I can represent simple nuclear processes using balanced nuclear equations.Syllabus
Syllabus 8867, 11(k). Represent simple nuclear processes using balanced nuclear equations.
I can apply conservation of nucleon number, charge and mass-energy in nuclear processes.Syllabus
Syllabus 8867, 11(l). Apply conservation of nucleon number, charge and mass-energy in nuclear processes.
Make a guess. You are not marked.
Uranium-238, proton number 92, emits an alpha particle. What are the nucleon and proton numbers of the daughter nucleus?
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Nucleon number 234 and proton number 90.
An alpha particle is 4 nucleons and 2 protons. 238 - 4 = 234 and 92 - 2 = 90, which is thorium.
A nuclear equation accounts for nucleon number and electric charge. Balance both before identifying an unknown product; energy and momentum must also be conserved in the complete process.
In , A counts nucleons and Z counts protons. For an emitted particle, the lower number records its charge in units of +e. An electron therefore has lower number -1 even though it contains no negative proton.
Alpha emission
An alpha particle carries four nucleons and charge +2e. The daughter therefore has A reduced by 4 and Z reduced by 2. Here 238 = 234 + 4 and 92 = 90 + 2.
Beta-minus emission
This displays the nucleon and charge account: 14 = 14 + 0 and 6 = 7 - 1. Within the nucleus, a neutron changes into a proton. The total nucleon number stays the same, while the daughter's proton number rises by one.
The emitted electron is produced by the nuclear process, rather than removed from an atomic shell. The displayed A/Z account is not a claim that only two products carry energy and momentum: an additional uncharged particle also participates.
Gamma emission
The star indicates a higher-energy nuclear state. A gamma photon carries away energy and momentum; the nucleus has the same A and Z afterwards. The energy change does not require a change of element or isotope.
A reaction can have an incoming particle
Both sides contain 18 nucleons and charge +9e. A and Z balance across all incoming and outgoing particles, not just the two nuclei that look most prominent.
Conserved nucleon number does not mean conserved rest mass. A change of total rest mass can accompany a change in kinetic or radiation energy while total mass-energy remains conserved.
Check your understanding
In beta-minus decay, a parent nucleus has nucleon number A and proton number Z. What are the daughter values in the nucleon/charge account?
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A and Z + 1.
A neutron-to-proton change leaves the total nucleon count unchanged. The new proton raises nuclear Z by one, while the emitted electron carries charge number -1.