Why do heavy elements such as uranium undergo fission while light elements such as hydrogen and lithium undergo fusion?

Nuclear fission is a type of reaction where a heavy nucleus is split into two or more smaller nuclei, often accompanied by the release of neutrons and a vast amount of energy.

In the nuclei of heavy elements like uranium, the protons are tightly packed, resulting in strong repulsive forces. This electric repulsion competes with the attractive nuclear force that holds the nucleus together. When the balance tilts because this attraction cannot sufficiently counteract the repulsion, the nucleus becomes unstable and might absorb a free neutron. This absorption can provide enough additional energy to cause the nucleus to split, releasing more neutrons and a surge of energy.

This process is critical for powering nuclear reactors and atomic bombs. It is facilitated by substances called moderators, which slow down the emitted neutrons to sustain a controlled chain reaction – the principal operation of a nuclear power plant.

In stark contrast to fission, nuclear fusion involves the combining of small, light nuclei to form a larger nucleus. Typical fusion reactions fuse hydrogen isotopes to form helium- - a process that powers the stars, including our sun.

In lighter elements, protons and neutrons are in closer proximity, meaning the strong nuclear force dominates over repulsive forces. Under extreme conditions of high temperature and pressure, as found in stars or hydrogen bombs, these nuclei can overcome their electrostatic repulsion to fuse together. During fusion, energy is released according to the mass-energy equivalence principle (E=mc²), as the mass of the resulting nucleus is less than the sum of its parts.

Fusion energy is a highly sought-after goal for its potential as a clean and almost limitless source of power. However, reaching and controlling the conditions for sustained fusion on Earth presents significant engineering and scientific challenges.

The atomic binding energy curve is a graph that shows the average binding energy per nucleon as a function of atomic mass. It provides valuable insights into why certain elements undergo fission or fusion.

The curve highlights that the most stable nucleus is that of iron-56; thus atomic nuclei will tend to move towards this peak of stability. Elements heavier than iron have a high binding energy, enabling them to release energy when split – a process known as fission. Conversely, lighter elements, being further away from the peak, can release energy when they combine – known as fusion.

This curve is fundamental in understanding nuclear transmutation processes, such as those within stars, where lighter elements fuse to form heavier ones until they reach iron, beyond which energy can be liberated through fission in supernova events.

Nuclear force, or strong force, is one of the four fundamental forces of nature and is responsible for holding protons and neutrons together in an atomic nucleus.

This force is incredibly strong, but it acts over very short ranges — on the order of the size of atomic nuclei. It must counterbalance the electrostatic repulsion between positively charged protons. In light nuclei, the nuclear force can contain these repulsive forces effectively, facilitating fusion when conditions are right. However, as nuclei get larger, the effective range of the nuclear force becomes an issue and the nucleus can become unstable, setting the stage for fission to occur.

Understanding the nuclear force is critical not only in studying nuclear reactions but also in the broader context of particle physics and the formation of matter in the universe.