14.9: Nuclear Binding Energy
Explain trends in the relative stability of nuclei.
As a simple example of the energy associated with the strong nuclear force, consider the helium atom composed of two protons, two neutrons, and two electrons. The total mass of these six subatomic particles may be calculated as:
\[\underbrace{(2 \times 1.0073 \, \text{amu})}_{\text{protons}} + \underbrace{(2 \times 1.0087\, \text{amu})}_{\text{neutrons}} + \underbrace{(2 \times 0.00055\, \text{amu})}_{\text{electrons}} =4.0331\, \text{amu} \nonumber \]
However, mass spectrometric measurements reveal that the mass of an atom is 4.0026 amu, less than the combined masses of its six constituent subatomic particles. This difference between the calculated and experimentally measured masses is known as the mass defect of the atom. In the case of helium, the mass defect indicates a “loss” in mass of 4.0331 amu – 4.0026 amu = 0.0305 amu. The loss in mass accompanying the formation of an atom from protons, neutrons, and electrons is due to the conversion of that mass into energy that is evolved as the atom forms. The nuclear binding energy is the energy produced when the atoms’ nucleons are bound together; this is also the energy needed to break a nucleus into its constituent protons and neutrons. In comparison to chemical bond energies, nuclear binding energies are vastly greater, as we will learn in this section. Consequently, the energy changes associated with nuclear reactions are vastly greater than are those for chemical reactions.
The conversion between mass and energy is most identifiably represented by the mass-energy equivalence equation covered earlier, which results in massive amounts of energy for even tiny amounts of mass. Because the energy changes for breaking and forming bonds are so small compared to the energy changes for breaking or forming nuclei, the changes in mass during all ordinary chemical reactions are virtually undetectable. The most energetic chemical reactions exhibit enthalpies on the order of thousands of kJ/mol, which is equivalent to mass differences in the nanogram range (10 –9 g). On the other hand, nuclear binding energies are typically on the order of billions of kJ/mol, corresponding to mass differences in the milligram range (10 –3 g).
The relative stability of a nucleus is correlated with its binding energy per nucleon , the total binding energy for the nucleus divided by the number or nucleons in the nucleus. As we shall see, nuclear reactions will occur that lead towards maximizing the nuclear binding energy per nucleon. Whether this results in increasing the number of nucleons or decreasing the number of nucleons depends upon where the nucelus is on the curve shown in Figure \(\PageIndex{1}\). In the subsections which follow, we will explore each of these possibilities in more detail.
Glossary
- mass defect
- The difference between the calculated and experimentally measured masses.
- nuclear binding energy
- The energy needed to break a nucleus into its constituent protons and neutrons.
- binding energy per nucleon
- The total binding energy for the nucleus divided by the number or nucleons in the nucleus.