Episode 525: Binding energy
Lesson for 16-19
- Activity time 100 minutes
- Level Advanced
This episode discusses: mass defect and atomic mass units; and, fission and fusion linked to binding energy. And is accompanied by worked examples, student question sets, and student activities.
- Discussion: Introducing mass defect and atomic mass units (10 minutes)
- Discussion: Mass defect and binding energy (10 minutes)
- Worked example: Calculating binding energy (10 minutes)
- Student questions: Calculations (20 minutes)
- Student activity: Spreadsheet calculations (20 minutes)
- Student activity: Spreadsheet calculations of binding energy per nucleon (20 minutes)
- Discussion: Fission and fusion linked to binding energy graph (10 minutes)
Discussion: Introducing mass defect and atomic mass units
Ask your students to consider whether the following data is self-consistent:
proton mass, mp = 1.673 × 10-27 kg
neutron mass, mn = 1.675 × 10-27 kg
mass of a 42He nucleus = 6.643 × 10-27 kg
The mass of a 42He nucleus is less than the sum of the masses of its parts; this is true for all nuclides. So much for conservation of mass.
Introduce the atomic mass unit (amu, or u) as a convenient unit of nuclear mass. 1 amu or 1 u, which is 112 the mass of a neutral 126C atom (i.e. including its six electrons):
u = 1.66056 × 10-27 kg.
mp = 1.0073 u
mn = 1.0087 u
me = 0.00055 u
mass of a neutral
atom = 4.0026 u
Discussion: Mass defect and binding energy
What has happened to the missing mass – or mass defect – between the whole and the sum of the parts? To separate the nucleons of a nucleus, work has to be done against the attractive strong nuclear force. The work done is known as the binding energy. If the nucleons are not bound, then it takes no energy to separate them; the binding energy when they are separated is zero. If protons and neutrons (nucleons) are bound together in a nucleus, the bound nucleus must have less rest energy than the rest energy of the nucleons of which it is made.
In turn, this means that the mass of the nucleus must be less than the sum of the masses of its nucleons.
Binding energy can be a rather confusing term because students often think that this means that energy is required to bind nucleons together. As with chemical bonds, this is the opposite of the truth. Energy is needed to break bonds, or separate nucleons.
Einstein’s Special Theory of Relativity (1905) relates mass and energy via the equation E = m × c 2 (where c is the speed of light in a vacuum). In this case, we have:
binding energy = mass defect × c 2
Δ E = Δ m × c 2
(It is not advisable to talk about mass being
converted to energy or similar expressions. It is better to say that, in measuring an object’s mass, we are determining its energy. A helium nucleus has less mass than its constituent nucleons; in pulling them apart, we do work and so give them energy; hence their mass is greater.)
Worked examples: Calculating binding energy
Calculate the mass defect and binding energy for 42He
(Mass defect = 0.053 × 10-27 kg;
binding energy = 1.59 × 10-12 J
binding energy = 9.94 MeV)
Student activity: A data analysis exercise using Excel
This uses a spreadsheet to calculate binding energy for a number of nuclides.
Another spreadsheet activity, this time looking at the binding energy per nucleon. Note that it is desirable to plot this graph with a negative energy axis; this means that the lowest values are for the most stable nuclides.
Briefly discuss fission and fusion in terms of the graph. Although the fission
jump looks quite small compared to a typical fusion jump, the graph is plotting BE per nucleon. Many more nucleons are involved in the fission of heavy atoms than in the fusion of lighter ones. (This topic can be developed further when discussing nuclear power.)