Nuclear Stability (DP IB Physics: HL): Revision Note

Nuclear Stability

• The most common elements in the universe all tend to have values of N and Z less than 20 (plus iron which has Z = 26, N = 30)

• Where:

  • N = number of neutrons

  • Z = number of protons / atomic number

• This is because lighter elements (with fewer protons) tend to be much more stable than heavier ones (with many protons)

• Nuclear stability becomes vastly clearer when viewed on a graph of N against Z

This nuclear stability curve shows the line of stable isotopes and which unstable isotopes will emit alpha or beta particles

• The line of stability shows N and Z values that produce stable nuclei

  • If a nucleus on this line were to have more neutrons, for example, it would move above the line and become an unstable β⁻ emitter

• A nucleus will be unstable if it has:

  • Too many neutrons

  • Too many protons

  • Too many nucleons ie. too heavy

  • Too much energy

• An unstable atom wants to become stable

• For light isotopes, Z < 20:

  • All these nuclei tend to be very stable

  • They follow the straight-line N = Z

• For heavy isotopes, Z > 20:

  • The neutron-proton ratio increases

  • Stable nuclei must have more neutrons than protons

Evidence for the Strong Nuclear Force

• The imbalance in the neutron-proton ratio is very significant to the stability of nuclei

• At a short range (around 1–3 fm), nucleons are bound by the strong nuclear force

• Below 1 fm, the strong nuclear force is repulsive in order to prevent the nucleus from collapsing

• At longer ranges, the electromagnetic force acts between protons, so more protons cause more instability

• Therefore, as more protons are added to the nucleus, more neutrons are needed to add distance between protons to reduce the electrostatic repulsion

• Also, the extra neutrons increase the amount of binding force which helps to bind the nucleons together