Why are heavy nuclei unstable compared to lighter ones?

Heavy nuclei are unstable compared to lighter ones due to the imbalance between nuclear forces and electrostatic repulsion.

In more detail, the stability of a nucleus is determined by the balance between two fundamental forces: the strong nuclear force, which binds protons and neutrons together, and the electrostatic force, which causes protons to repel each other. The strong nuclear force is much stronger than the electrostatic force, but its range is very short. Therefore, in small nuclei where all the nucleons are close together, the strong nuclear force dominates, making the nucleus stable.

However, as the nucleus gets larger, the situation changes. The strong nuclear force only acts over very short distances, roughly the size of a typical atomic nucleus. Beyond this range, it drops off rapidly and becomes negligible. On the other hand, the electrostatic force between protons, which is always repulsive, acts over much larger distances. This means that in a large nucleus, the protons on one side of the nucleus feel a significant electrostatic repulsion from the protons on the other side, but the strong nuclear force from the distant nucleons is too weak to counterbalance this repulsion.

This imbalance between the strong nuclear force and the electrostatic repulsion makes heavy nuclei unstable. They have a tendency to decay into smaller, more stable nuclei, a process known as radioactive decay. This can occur through various mechanisms, such as alpha decay (where the nucleus emits an alpha particle, consisting of two protons and two neutrons), beta decay (where a neutron turns into a proton or vice versa), or fission (where the nucleus splits into two smaller nuclei).

IB Physics Tutor Summary: Heavy nuclei are unstable because of an imbalance between two forces: the strong nuclear force, which binds particles in the nucleus, and the electrostatic force, which pushes protons apart. The strong nuclear force weakens over short distances, while electrostatic force extends further, making large nuclei unstable and prone to decay into smaller, stable forms.