Strong nuclear force
Remember that the strong nuclear force is essential to the existence of nuclei, as it overpowers the charge repulsion of protons to bind them together into one cohesive mass with neutrons. However, it is only tangible up to 2.0 fm, and its effect rapidly drops off after this. Like the other fundamental forces, you are expected to know the evidence behind this force. There are two fundamental pieces of evidence you need to be aware of:
- Nuclear binding energy – just as intermolecular forces, the creation of bonds releases energy whereas the breaking of bonds requires energy. The mass defect of nuclei is explained by the nuclear binding energy, which can only be caused by a bond between nucleons – the strong nuclear force.
- Rutherford scattering formula – this determines the scattered intensity due to electrostatic repulsion based on the energy of the fired alpha particles. However, high energy alpha particles deviate from this formula as they close in on the nucleus and are affected by the strong nuclear force, significantly decreasing the scattering intensity.
Nuclear stability
Whilst the strong nuclear force is powerful, as the number of protons in a nucleus increases (with increasing atomic number), the electrostatic repulsion between the protons also significantly increases. Neutrons are there to facilitate the strong nuclear force whilst spacing out protons to overall reducing the strength of the electrostatic repulsion and keep the nucleus together.
The stability of a nucleus has been tried to be explained by a constant ratio of neutrons-to-protons (N/Z ratio), but for stable nuclei of an increasing size, the neutron-proton ratio increases too. This is because:
- As nuclear size increases, more protons come into close proximity with one another.
- The built-up electrostatic repulsion outweighs the strong nuclear force if the ratio is 1:1.
- Thus, proton density must decrease as nuclear size increases to maintain stability.
- This results in an increase in the neutron-proton ratio.
Nuclear energy levels
The concept of nuclear stability suggests that nuclei possess specific energy levels. When viewing the nuclear binding energy curve, a few things are observable:
- From mass numbers 1 to 30, there is a great increase in nuclear binding energy per nucleon. This is due to increased forces per nucleon, causing a more tightly bound nucleus and releasing more energy.
- From mass numbers 30 to 60, the nuclear binding energy per nucleon decreases vastly as the forces per nucleon increase minorly.
- From mass 60 onwards, the binding curve is approximately stable. This is because the nucleus is now so large that nuclear force don’t extend across the whole radius, meaning that the electrostatic forces and strong nuclear force cancel out with each new nucleon.
- At the tail end of the curve, the heavier elements have a gradual decrease in binding energy per nucleon. This is because the nucleus is now so large that the strong nuclear force is not substantial enough to keep the nucleus together and so electrostatic repulsion begins to dominate more.