Mass Defect & Binding Energy
A bound nucleus has less mass than the same protons and neutrons when they are separated. This single comparison gives both mass defect and binding energy.
The nucleons remain; the system mass changes
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The left and right sides contain the same nucleons. No proton or neutron is missing. The bound system has lower energy, so it also has lower mass.
Definition
2 marksWhat is meant by the mass defect of a nucleus?
Model answer: The mass defect is the difference between the total mass of the constituent nucleons when separated to infinity and the mass of the nucleus.
(9702/42/O/N/25 Q9(a))
This form uses proton, neutron and nuclear masses. If a question gives atomic masses, keep the electron accounting consistent instead of inserting this form automatically.
Binding energy is the energy needed to separate the nucleus
Imagine reversing nucleus formation. To separate all nucleons, energy must be supplied against the attractive nuclear force. The minimum required energy is the binding energy.
Definition
2 marksWhat is meant by the binding energy of a nucleus?
Model answer: The binding energy is the minimum energy required to separate the nucleus completely into its individual nucleons at infinity.
(9702/42/F/M/24 Q8(a))
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| Quantity | Meaning | Unit | Use |
|---|---|---|---|
| Mass defect | Mass difference between separated and bound states | kg or u | Convert mass to energy |
| Binding energy | Energy needed to separate the whole nucleus | J or MeV | Describe the whole nucleus |
| Binding energy per nucleon | Average binding energy for one nucleon | J or MeV per nucleon | Compare nuclei of different sizes |
Common mistake
A nucleus has a mass defect of kg. What physical quantity is found by multiplying this value by the square of ?
Show worked answer
The binding energy of the nucleus: the minimum energy needed to separate it completely into individual nucleons at infinity.