Mass Defect & Nuclear Binding Energy
Energy & Mass Equivalence
- Einstein showed in his Theory of Relativity that matter can be considered a form of energy and hence, he proposed:
- Mass can be converted into energy
- Energy can be converted into mass
- This is known as mass-energy equivalence, and can be summarised by the equation:
E = mc2
Mass Defect & Binding Energy
- Experiments into nuclear structure have found that the total mass of a nucleus is less than the sum of the masses of its constituent nucleons
- This difference in mass is known as the mass defect
- Mass defect is defined as:
The difference between an atom's mass and the sum of the masses of its protons and neutrons
- The mass defect Δm of a nucleus can be calculated using:
Δm = Zmp + (A – Z)mn – mtotal
- Where:
- Z = proton number
- A = nucleon number
- mp = mass of a proton (kg)
- mn = mass of a neutron (kg)
- mtotal = measured mass of the nucleus (kg)
A system of separated nucleons has a greater mass than a system of bound nucleons
- Due to the equivalence of mass and energy, this decrease in mass implies that energy is released in the process
- Since nuclei are made up of neutrons and protons, there are forces of repulsion between the positive protons
- Therefore, it takes energy, ie. the binding energy, to hold nucleons together as a nucleus
- Binding energy is defined as:
The energy released when a nucleus forms from constituent nucleons
OR
The (minimum) energy needed to break a nucleus up into its constituent nucleons (protons and neutrons)
- Energy and mass are proportional, so, the total energy of a nucleus is less than the sum of the energies of its constituent nucleons
- The formation of a nucleus from a system of isolated protons and neutrons is therefore an exothermic reaction - meaning that it releases energy
- This can be calculated using the equation:
E = Δmc2
Examiner Tip
Avoid describing the binding energy as the energy stored in the nucleus – this is not correct – it is energy that must be put into the nucleus to pull it apart.