3.8.1.6
Mass defect is the difference between a nucleus and its separated nucleons
Nuclear Energy & Binding Energy — AQA A-Level Physics
Key Definition
Mass defect — The difference between the total mass of the separated nucleons and the measured mass of the nucleus. Symbol: Δm.
$$\Delta m = Zm_p + (A - Z)m_n - m_{\text{total}}$$
- $Δm$: mass defectThe difference between the total mass of the individual nucleons and the actual mass of the nucleus. This mass is converted to binding energyThe capacity to do work. Measured in joules (J).. (kg or u)
- $Z$: proton number
- $A$: nucleon number
- $mₚ$: mass of one proton
- $mₙ$: mass of one neutron
- $m_total$: measured mass of the nucleus
- The total mass of a nucleus is always LESS than the sum of its individual nucleon masses.
- This 'missing' mass has been converted to the binding energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). required to completely separate a nucleus into its individual protons and neutrons. Equal to the mass defectThe difference between the total mass of the individual nucleons and the actual mass of the nucleus. This mass is converted to binding energy. multiplied by c². that holds the nucleus together.
- A system of separated nucleons has greater mass than the same nucleons bound in a nucleus.
Common Mistake
MEDIUM
Students often: Describing mass defectThe difference between the total mass of the individual nucleons and the actual mass of the nucleus. This mass is converted to binding energy. as 'the decrease in mass during radioactive decayThe spontaneous and random disintegration of an unstable nucleus, emitting radiation (alpha, beta, or gamma) to become more stable.'.
Instead: Mass defect refers specifically to the difference between separated nucleons and the bound nucleus. The mass loss during decay is a separate (related) concept.
Instead: Mass defect refers specifically to the difference between separated nucleons and the bound nucleus. The mass loss during decay is a separate (related) concept.