How does nuclear fission differ from fusion?

Nuclear fission involves splitting a heavy nucleus into two lighter ones, while fusion combines two light nuclei into a heavier one.

Nuclear fission and fusion are two different types of reactions that release energy due to the interplay of the strong nuclear force, which binds protons and neutrons, and the electromagnetic force, which makes protons repel each other. In nuclear fission, a heavy nucleus, such as uranium or plutonium, is split into two or more lighter nuclei, along with the release of a large amount of energy. This process is typically initiated by the absorption of a neutron by the heavy nucleus, which makes it unstable and causes it to split.

On the other hand, nuclear fusion involves two light nuclei, typically isotopes of hydrogen like deuterium and tritium, coming together to form a heavier nucleus, such as helium. This process also releases a large amount of energy, but it requires extremely high temperatures and pressures to overcome the electromagnetic repulsion between the positively charged nuclei. This is why fusion reactions are common in the sun and other stars, which have the necessary conditions.

The energy released in both fission and fusion reactions is much greater than that in chemical reactions, because it involves changes in the nucleus of the atom, which holds most of the atom's mass. This is explained by Einstein's mass-energy equivalence principle, E=mc^2, which states that a small amount of mass can be converted into a large amount of energy.

However, there are significant differences in the practical applications of fission and fusion. Fission is currently used in nuclear power plants and nuclear weapons, because it can be easily initiated and controlled. Fusion, despite its potential for providing a nearly limitless and clean source of energy, is still not commercially viable due to the difficulty in achieving and maintaining the conditions necessary for a sustained reaction.