Mass Defect and Energy

Question 1

Electron Volts and Joules

Answer 1

When we talk about energy, we use the SI Unit Joules. Nuclear reactions are much smaller, so a smaller unit of energy called the electron volt is used.

1eV = 1.6 * 10^-19

Question 2

Mass Defect

Answer 2

If we compare the mass of individual protons and neutrons to the mass of a nucleus with the same number of protons and neutrons, we will find that the nucleus has less mass. To calculate mass defect =

mass of individual nucleons (particles that make up nucleus) - mass of nucleus

Question 3

Atomic Mass Units

Answer 3

When calculating mass defect, we can use kg or use atomic mass units. Using amu can be useful because the calculated values are easier to work with for such small values (formula sheet).

Question 4

Binding Energy

Answer 4

The binding energy is the amount holding a nucleus together, which is found using the mass defect. Another way of thinking about binding energy is the amount of energy it would take to split the nucleus into individual nucleons.

Binding energy and mass defect are proportional: nuclei with higher mass defect have higher binding energy, nuclei with lower mass defect have lower binding energy.

Question 5

Binding Energy Per Nucleon

Answer 5

As the size of a nucleus increases, the binding energy increases (due to greater strong nuclear force). Because of this, binding energy itself does not tell us much. It is much more useful to look at binding energy per nucleon. This is a better way of comparing binding energies across different types of nuclei.

To do this, you could calculate binding energy and divide this number by the number of nucleons.

Question 6

Binding Energy per Nucleon: Fission VS Fusion

Answer 6

The binding energy curve EXPLAINS which atoms will undergo nuclear fission or fusion.

Area of stability: contains the most stable nuclei (the more binding energy per nucleon, the more stable it is). Before stability region: smaller nuclei - these nuclei would increase binding energy by undergoing nuclear fusion (in order to become more stable). This section of the graph has an upward slope. After stability region: larger nuclei - these nuclei would increase binding energy by undergoing nuclear fission. Downwards slope. This occurs because a point is reached where it does not matter how many nucleons we add one nucleon with, because the nuclear strong force is only acting on a short range, eventually the outer nucleons don’t have an effect, if anything it means that the counterforce (electromagnetic force, repels protons), starts to contribute to nuclear instability. Therefore we reach a point where the larger an atom is the less stable it becomes, because the electromagnetic rorce tries to fight against the strong nuclear force. At this point, we get larger nuclei but less average binding energy per nucleon as our nucleons still increase.